diff --git a/lib/api/core.dart b/lib/api/core.dart
index 3a56cc4db90..ae52c716309 100644
--- a/lib/api/core.dart
+++ b/lib/api/core.dart
@@ -1,6 +1,7 @@
 import 'dart:convert';
 import 'dart:io';
 
+import 'package:flutter_gen/gen_l10n/zulip_localizations.dart';
 import 'package:http/http.dart' as http;
 
 import '../log.dart';
@@ -84,15 +85,25 @@ class ApiConnection {
     try {
       response = await _client.send(request);
     } catch (e) {
-      final String message;
+      final String? message;
+      final String Function(ZulipLocalizations)? translatableMessage;
       if (e is http.ClientException) {
         message = e.message;
+        translatableMessage = null;
       } else if (e is TlsException) {
         message = e.message;
+        translatableMessage = null;
       } else {
-        message = 'Network request failed';
+        message = null;
+        translatableMessage = (ZulipLocalizations zulipLocalizations) {
+          return zulipLocalizations.apiConnectionNetworkRequestFailed;
+        };
       }
-      throw NetworkException(routeName: routeName, cause: e, message: message);
+      throw NetworkException(
+        routeName: routeName,
+        cause: e,
+        message: message,
+        translatableMessage: translatableMessage);
     }
 
     final int httpStatus = response.statusCode;
@@ -173,6 +184,7 @@ ApiRequestException _makeApiException(String routeName, int httpStatus, Map<Stri
         // TODO(server): `code` should always be present.  Get the "Invalid API key" case fixed.
         code: json.remove('code') ?? 'BAD_REQUEST',
         message: json.remove('msg'),
+        translatableMessage: null,
         data: json,
       );
     }
diff --git a/lib/api/exception.dart b/lib/api/exception.dart
index 43f548e6dfa..dc8d826c503 100644
--- a/lib/api/exception.dart
+++ b/lib/api/exception.dart
@@ -1,4 +1,6 @@
 
+import 'package:flutter_gen/gen_l10n/zulip_localizations.dart';
+
 /// Some kind of error from a Zulip API network request.
 sealed class ApiRequestException implements Exception {
   /// The name of the Zulip API route for the request.
@@ -11,13 +13,26 @@ sealed class ApiRequestException implements Exception {
   /// and its route name would be "getMessages".
   final String routeName;
 
-  /// A user-facing description of the error.
+  /// Server supplied user-facing description of the error.
+  ///
+  /// For [ZulipApiException] this may be supplied by the server as the `message`
+  /// property in the JSON response.
+  // TODO(i18n): no method yet to translate a server provided string.
+  final String? message;
+
+  /// App supplied user-facing description of the error.
   ///
-  /// For [ZulipApiException] this is supplied by the server as the `message`
-  /// property in the JSON response, and is translated into the user's language.
-  final String message;
+  /// This description will be translated according to the
+  /// current set locale.
+  final String Function(ZulipLocalizations)? translatableMessage;
+
+  ApiRequestException({required this.routeName, required this.message, required this.translatableMessage});
 
-  ApiRequestException({required this.routeName, required this.message});
+  String toTranslatedString(ZulipLocalizations zulipLocalizations) {
+    return (translatableMessage != null)
+      ? translatableMessage!(zulipLocalizations)
+      : super.toString();
+  }
 }
 
 /// An error returned through the Zulip server API.
@@ -46,6 +61,7 @@ class ZulipApiException extends ApiRequestException {
     required this.httpStatus,
     required this.data,
     required super.message,
+    required super.translatableMessage,
   }) : assert(400 <= httpStatus && httpStatus <= 499);
 }
 
@@ -58,7 +74,7 @@ class NetworkException extends ApiRequestException {
   /// but empirically it can be [TlsException] and possibly others.
   final Object cause;
 
-  NetworkException({required super.routeName, required super.message, required this.cause});
+  NetworkException({required super.routeName, required super.message, required super.translatableMessage, required this.cause});
 }
 
 /// Some kind of server-side error in handling the request.
@@ -79,6 +95,7 @@ abstract class ServerException extends ApiRequestException {
     required this.httpStatus,
     required this.data,
     required super.message,
+    required super.translatableMessage,
   });
 }
 
@@ -89,7 +106,11 @@ class Server5xxException extends ServerException {
     required super.httpStatus,
     required super.data,
   }) : assert(500 <= httpStatus && httpStatus <= 599),
-       super(message: 'Network request failed: HTTP status $httpStatus'); // TODO(i18n)
+       super(
+         message: null,
+         translatableMessage: (ZulipLocalizations zulipLocalizations) {
+           return zulipLocalizations.serverExceptionRequestFailed(httpStatus);
+         });
 }
 
 /// An error where the server's response doesn't match the Zulip API.
@@ -110,5 +131,9 @@ class MalformedServerResponseException extends ServerException {
     required super.routeName,
     required super.httpStatus,
     required super.data,
-  }) : super(message: 'Server gave malformed response; HTTP status $httpStatus'); // TODO(i18n)
+  }) : super(
+         message: null,
+         translatableMessage: (ZulipLocalizations zulipLocalizations) {
+           return zulipLocalizations.serverExceptionMalformedResponse(httpStatus);
+         });
 }
diff --git a/lib/widgets/action_sheet.dart b/lib/widgets/action_sheet.dart
index f7bee7b7f23..e5d815a5036 100644
--- a/lib/widgets/action_sheet.dart
+++ b/lib/widgets/action_sheet.dart
@@ -1,5 +1,6 @@
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
+import 'package:flutter_gen/gen_l10n/zulip_localizations.dart';
 import 'package:share_plus/share_plus.dart';
 
 import '../api/exception.dart';
@@ -43,7 +44,7 @@ abstract class MessageActionSheetMenuItemButton extends StatelessWidget {
   }) : assert(messageListContext.findAncestorWidgetOfExactType<MessageListPage>() != null);
 
   IconData get icon;
-  String get label;
+  String Function(ZulipLocalizations) get translateLabel;
   void Function(BuildContext) get onPressed;
 
   final Message message;
@@ -51,10 +52,11 @@ abstract class MessageActionSheetMenuItemButton extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
+    final zulipLocalizations = ZulipLocalizations.of(context);
     return MenuItemButton(
       leadingIcon: Icon(icon),
       onPressed: () => onPressed(context),
-      child: Text(label));
+      child: Text(translateLabel(zulipLocalizations)));
   }
 }
 
@@ -67,7 +69,9 @@ class ShareButton extends MessageActionSheetMenuItemButton {
 
   @override get icon => Icons.adaptive.share;
 
-  @override get label => 'Share';
+  @override get translateLabel => (ZulipLocalizations zulipLocalizations) {
+    return zulipLocalizations.actionSheetShare;
+  };
 
   @override get onPressed => (BuildContext context) async {
     // Close the message action sheet; we're about to show the share
@@ -104,22 +108,23 @@ Future<String?> fetchRawContentWithFeedback({
     // - If request(s) take(s) a long time, show snackbar with cancel
     //   button, like "Still working on quote-and-reply…".
     //   On final failure or success, auto-dismiss the snackbar.
+    final zulipLocalizations = ZulipLocalizations.of(context);
     try {
       fetchedMessage = await getMessageCompat(PerAccountStoreWidget.of(context).connection,
         messageId: messageId,
         applyMarkdown: false,
       );
       if (fetchedMessage == null) {
-        errorMessage = 'That message does not seem to exist.';
+        errorMessage = zulipLocalizations.actionSheetMessageDoesNotSeemToExist;
       }
     } catch (e) {
       switch (e) {
         case ZulipApiException():
-          errorMessage = e.message;
+          errorMessage = e.toTranslatedString(zulipLocalizations);
         // TODO specific messages for common errors, like network errors
         //   (support with reusable code)
         default:
-          errorMessage = 'Could not fetch message source.';
+          errorMessage = zulipLocalizations.actionSheetCouldNotFetchMessageSource;
       }
     }
 
@@ -146,12 +151,15 @@ class QuoteAndReplyButton extends MessageActionSheetMenuItemButton {
 
   @override get icon => Icons.format_quote_outlined;
 
-  @override get label => 'Quote and reply';
+  @override get translateLabel => (ZulipLocalizations zulipLocalizations) {
+    return zulipLocalizations.actionSheetQuoteAndReply;
+  };
 
   @override get onPressed => (BuildContext bottomSheetContext) async {
     // Close the message action sheet. We'll show the request progress
     // in the compose-box content input with a "[Quoting…]" placeholder.
     Navigator.of(bottomSheetContext).pop();
+    final zulipLocalizations = ZulipLocalizations.of(messageListContext);
 
     // This will be null only if the compose box disappeared after the
     // message action sheet opened, and before "Quote and reply" was pressed.
@@ -174,7 +182,7 @@ class QuoteAndReplyButton extends MessageActionSheetMenuItemButton {
     final rawContent = await fetchRawContentWithFeedback(
       context: messageListContext,
       messageId: message.id,
-      errorDialogTitle: 'Quotation failed',
+      errorDialogTitle: zulipLocalizations.actionSheetQuotationFailed,
     );
 
     if (!messageListContext.mounted) return;
@@ -203,26 +211,29 @@ class CopyButton extends MessageActionSheetMenuItemButton {
 
   @override get icon => Icons.copy;
 
-  @override get label => 'Copy message text';
+  @override get translateLabel => (ZulipLocalizations zulipLocalizations) {
+    return zulipLocalizations.actionSheetCopyMessageText;
+  };
 
   @override get onPressed => (BuildContext context) async {
     // Close the message action sheet. We won't be showing request progress,
     // but hopefully it won't take long at all, and
     // fetchRawContentWithFeedback has a TODO for giving feedback if it does.
     Navigator.of(context).pop();
+    final zulipLocalizations = ZulipLocalizations.of(messageListContext);
 
     final rawContent = await fetchRawContentWithFeedback(
       context: messageListContext,
       messageId: message.id,
-      errorDialogTitle: 'Copying failed',
+      errorDialogTitle: zulipLocalizations.actionSheetCopyingFailed,
     );
 
     if (rawContent == null) return;
 
     if (!messageListContext.mounted) return;
 
-    // TODO(i18n)
-    copyWithPopup(context: context, successContent: const Text('Message copied'),
+    copyWithPopup(context: context,
+      successContent: Text(zulipLocalizations.actionSheetMessageCopied),
       data: ClipboardData(text: rawContent));
   };
 }
diff --git a/lib/widgets/compose_box.dart b/lib/widgets/compose_box.dart
index dbcca82903f..136ca0e89d6 100644
--- a/lib/widgets/compose_box.dart
+++ b/lib/widgets/compose_box.dart
@@ -5,6 +5,7 @@ import 'package:flutter/services.dart';
 import 'package:flutter_gen/gen_l10n/zulip_localizations.dart';
 import 'package:image_picker/image_picker.dart';
 
+import '../api/exception.dart';
 import '../api/model/model.dart';
 import '../api/route/messages.dart';
 import '../model/compose.dart';
@@ -48,12 +49,12 @@ enum TopicValidationError {
   mandatoryButEmpty,
   tooLong;
 
-  String message() {
+  String translateMessage(ZulipLocalizations zulipLocalizations) {
     switch (this) {
       case tooLong:
-        return "Topic length shouldn't be greater than 60 characters.";
+        return zulipLocalizations.topicValidationErrorTooLong;
       case mandatoryButEmpty:
-        return 'Topics are required in this organization.';
+        return zulipLocalizations.topicValidationErrorMandatoryButEmpty;
     }
   }
 }
@@ -90,16 +91,16 @@ enum ContentValidationError {
   quoteAndReplyInProgress,
   uploadInProgress;
 
-  String message() {
+  String translateMessage(ZulipLocalizations zulipLocalizations) {
     switch (this) {
       case ContentValidationError.tooLong:
-        return "Message length shouldn't be greater than 10000 characters.";
+        return zulipLocalizations.contentValidationErrorTooLong;
       case ContentValidationError.empty:
-        return 'You have nothing to send!';
+        return zulipLocalizations.contentValidationErrorEmpty;
       case ContentValidationError.quoteAndReplyInProgress:
-        return 'Please wait for the quotation to complete.';
+        return zulipLocalizations.contentValidationErrorQuoteAndReplyInProgress;
       case ContentValidationError.uploadInProgress:
-        return 'Please wait for the upload to complete.';
+        return zulipLocalizations.contentValidationErrorUploadInProgress;
     }
   }
 }
@@ -208,10 +209,10 @@ class ComposeContentController extends ComposeController<ContentValidationError>
   ///
   /// Returns an int "tag" that should be passed to registerUploadEnd on the
   /// upload's success or failure.
-  int registerUploadStart(String filename) {
+  int registerUploadStart(String filename, ZulipLocalizations zulipLocalizations) {
     final tag = _nextUploadTag;
     _nextUploadTag += 1;
-    final placeholder = inlineLink('Uploading $filename...', null); // TODO(i18n)
+    final placeholder = inlineLink(zulipLocalizations.composeBoxUploadingFilename(filename), null);
     _uploads[tag] = (filename: filename, placeholder: placeholder);
     notifyListeners(); // _uploads change could affect validationErrors
     value = value.replaced(insertionIndex(), '$placeholder\n\n');
@@ -359,12 +360,14 @@ class _StreamContentInputState extends State<_StreamContentInput> {
   @override
   Widget build(BuildContext context) {
     final store = PerAccountStoreWidget.of(context);
-    final streamName = store.streams[widget.narrow.streamId]?.name ?? '(unknown stream)';
+    final zulipLocalizations = ZulipLocalizations.of(context);
+    final streamName = store.streams[widget.narrow.streamId]?.name
+      ?? zulipLocalizations.composeBoxUnknownStreamName;
     return _ContentInput(
       narrow: widget.narrow,
       controller: widget.controller,
       focusNode: widget.focusNode,
-      hintText: "Message #$streamName > $_topicTextNormalized");
+      hintText: zulipLocalizations.composeBoxStreamContentHint(streamName, _topicTextNormalized));
   }
 }
 
@@ -380,23 +383,25 @@ class _FixedDestinationContentInput extends StatelessWidget {
   final FocusNode focusNode;
 
   String _hintText(BuildContext context) {
+    final zulipLocalizations = ZulipLocalizations.of(context);
     switch (narrow) {
       case TopicNarrow(:final streamId, :final topic):
         final store = PerAccountStoreWidget.of(context);
-        final streamName = store.streams[streamId]?.name ?? '(unknown stream)';
-        return "Message #$streamName > $topic";
+        final streamName = store.streams[streamId]?.name
+          ?? zulipLocalizations.composeBoxUnknownStreamName;
+        return zulipLocalizations.composeBoxStreamContentHint(streamName, topic);
 
       case DmNarrow(otherRecipientIds: []): // The self-1:1 thread.
-        return "Jot down something";
+        return zulipLocalizations.composeBoxSelfDMHint;
 
       case DmNarrow(otherRecipientIds: [final otherUserId]):
         final store = PerAccountStoreWidget.of(context);
         final fullName = store.users[otherUserId]?.fullName;
-        if (fullName == null) return 'Type a message';
-        return 'Message @$fullName';
+        if (fullName == null) return zulipLocalizations.composeBoxDMHintNoName;
+        return zulipLocalizations.composeBoxDMHint(fullName);
 
       case DmNarrow(): // A group DM thread.
-        return 'Message group';
+        return zulipLocalizations.composeBoxGroupDMHint;
     }
   }
 
@@ -430,6 +435,7 @@ Future<void> _uploadFiles({
 }) async {
   assert(context.mounted);
   final store = PerAccountStoreWidget.of(context);
+  final zulipLocalizations = ZulipLocalizations.of(context);
 
   final List<_File> tooLargeFiles = [];
   final List<_File> rightSizeFiles = [];
@@ -445,18 +451,18 @@ Future<void> _uploadFiles({
     final listMessage = tooLargeFiles
       .map((file) => '${file.filename}: ${(file.length / (1 << 20)).toStringAsFixed(1)} MiB')
       .join('\n');
-    showErrorDialog( // TODO(i18n)
+    showErrorDialog(
       context: context,
-      title: 'File(s) too large',
-      message:
-        '${tooLargeFiles.length} file(s) are larger than the server\'s limit'
-        ' of ${store.maxFileUploadSizeMib} MiB and will not be uploaded:'
-        '\n\n$listMessage');
+      title: zulipLocalizations.composeBoxFilesTooLargeTitle(tooLargeFiles.length),
+      message: zulipLocalizations.composeBoxFilesTooLargeMessage(
+        tooLargeFiles.length,
+        store.maxFileUploadSizeMib,
+        listMessage));
   }
 
   final List<(int, _File)> uploadsInProgress = [];
   for (final file in rightSizeFiles) {
-    final tag = contentController.registerUploadStart(file.filename);
+    final tag = contentController.registerUploadStart(file.filename, zulipLocalizations);
     uploadsInProgress.add((tag, file));
   }
   if (!contentFocusNode.hasFocus) {
@@ -474,8 +480,12 @@ Future<void> _uploadFiles({
       if (!context.mounted) return;
       // TODO(#37): Specifically handle `413 Payload Too Large`
       // TODO(#37): On API errors, quote `msg` from server, with "The server said:"
+      final String message = (e is ApiRequestException)
+        ? e.toTranslatedString(zulipLocalizations)
+        : e.toString();
       showErrorDialog(context: context,
-        title: 'Failed to upload file: $filename', message: e.toString());
+        title: zulipLocalizations.composeBoxFailedToUploadFile(filename),
+        message: message);
     } finally {
       contentController.registerUploadEnd(tag, url);
     }
@@ -489,7 +499,6 @@ abstract class _AttachUploadsButton extends StatelessWidget {
   final FocusNode contentFocusNode;
 
   IconData get icon;
-  String get tooltip;
 
   /// Request files from the user, in the way specific to this upload type.
   ///
@@ -500,6 +509,8 @@ abstract class _AttachUploadsButton extends StatelessWidget {
   /// return an empty [Iterable] after showing user feedback as appropriate.
   Future<Iterable<_File>> getFiles(BuildContext context);
 
+  String getTooltip(ZulipLocalizations zulipLocalizations);
+
   void _handlePress(BuildContext context) async {
     final files = await getFiles(context);
     if (files.isEmpty) {
@@ -521,15 +532,17 @@ abstract class _AttachUploadsButton extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
+    final zulipLocalizations = ZulipLocalizations.of(context);
     return IconButton(
       icon: Icon(icon),
-      tooltip: tooltip,
+      tooltip: getTooltip(zulipLocalizations),
       onPressed: () => _handlePress(context));
   }
 }
 
 Future<Iterable<_File>> _getFilePickerFiles(BuildContext context, FileType type) async {
   FilePickerResult? result;
+  final zulipLocalizations = ZulipLocalizations.of(context);
   try {
     result = await FilePicker.platform
       .pickFiles(allowMultiple: true, withReadStream: true, type: type);
@@ -542,16 +555,21 @@ Future<Iterable<_File>> _getFilePickerFiles(BuildContext context, FileType type)
       // If the user hasn't checked "Don't ask again", they can always dismiss
       // our prompt and retry, and the permissions request will reappear,
       // letting them grant permissions and complete the upload.
-      showSuggestedActionDialog(context: context, // TODO(i18n)
-        title: 'Permissions needed',
-        message: 'To upload files, please grant Zulip additional permissions in Settings.',
-        actionButtonText: 'Open settings',
+      showSuggestedActionDialog(context: context,
+        title: zulipLocalizations.permissionsNeededTitle,
+        message: zulipLocalizations.permissionsReadExternalStorageDenied,
+        actionButtonText: zulipLocalizations.permissionsNeededOpenSettings,
         onActionButtonPress: () {
           AppSettings.openAppSettings();
         });
     } else {
-      // TODO(i18n)
-      showErrorDialog(context: context, title: 'Error', message: e.toString());
+      final String message = (e is ApiRequestException)
+        ? e.toTranslatedString(zulipLocalizations)
+        : e.toString();
+      showErrorDialog(
+        context: context,
+        title: zulipLocalizations.permissionsNeededError,
+        message: message);
     }
     return [];
   }
@@ -565,13 +583,14 @@ Future<Iterable<_File>> _getFilePickerFiles(BuildContext context, FileType type)
   });
 }
 
-class _AttachFileButton extends _AttachUploadsButton {
-  const _AttachFileButton({required super.contentController, required super.contentFocusNode});
+class _AttachFileButton extends _AttachUploadsButton { const _AttachFileButton({required super.contentController, required super.contentFocusNode});
 
   @override
   IconData get icon => Icons.attach_file;
+
   @override
-  String get tooltip => 'Attach files';
+  String getTooltip(ZulipLocalizations zulipLocalizations) =>
+    zulipLocalizations.composeBoxAttachFilesTooltip;
 
   @override
   Future<Iterable<_File>> getFiles(BuildContext context) async {
@@ -584,8 +603,10 @@ class _AttachMediaButton extends _AttachUploadsButton {
 
   @override
   IconData get icon => Icons.image;
+
   @override
-  String get tooltip => 'Attach images or videos';
+  String getTooltip(ZulipLocalizations zulipLocalizations) =>
+    zulipLocalizations.composeBoxAttachMediaTooltip;
 
   @override
   Future<Iterable<_File>> getFiles(BuildContext context) async {
@@ -599,8 +620,10 @@ class _AttachFromCameraButton extends _AttachUploadsButton {
 
   @override
   IconData get icon => Icons.camera_alt;
+
   @override
-  String get tooltip => 'Take a photo';
+  String getTooltip(ZulipLocalizations zulipLocalizations) =>
+      zulipLocalizations.composeBoxAttachFromCameraTooltip;
 
   @override
   Future<Iterable<_File>> getFiles(BuildContext context) async {
@@ -622,15 +645,20 @@ class _AttachFromCameraButton extends _AttachUploadsButton {
         // use a protected resource. After that, the only way the user can
         // grant it is in Settings.
         showSuggestedActionDialog(context: context,
-          title: zulipLocalizations.cameraAccessDeniedTitle,
-          message: zulipLocalizations.cameraAccessDeniedMessage,
-          actionButtonText: zulipLocalizations.cameraAccessDeniedButtonText,
+          title: zulipLocalizations.permissionsNeededTitle,
+          message: zulipLocalizations.permissionsNeededCameraAccessDenied,
+          actionButtonText: zulipLocalizations.permissionsNeededOpenSettings,
           onActionButtonPress: () {
             AppSettings.openAppSettings();
           });
       } else {
-        // TODO(i18n)
-        showErrorDialog(context: context, title: 'Error', message: e.toString());
+        final String message = (e is ApiRequestException)
+          ? e.toTranslatedString(zulipLocalizations)
+          : e.toString();
+        showErrorDialog(
+          context: context,
+          title: zulipLocalizations.permissionsNeededError,
+          message: message);
       }
       return [];
     }
@@ -699,15 +727,16 @@ class _SendButtonState extends State<_SendButton> {
 
   void _send() {
     if (_hasValidationErrors) {
+      final zulipLocalizations = ZulipLocalizations.of(context);
       List<String> validationErrorMessages = [
         for (final error in widget.topicController?.validationErrors ?? const [])
-          error.message(),
+          error.translateMessage(zulipLocalizations),
         for (final error in widget.contentController.validationErrors)
-          error.message(),
+          error.translateMessage(zulipLocalizations),
       ];
       showErrorDialog(
         context: context,
-        title: 'Message not sent',
+        title: zulipLocalizations.composeBoxMessageNotSent,
         message: validationErrorMessages.join('\n\n'));
       return;
     }
@@ -723,6 +752,7 @@ class _SendButtonState extends State<_SendButton> {
   Widget build(BuildContext context) {
     final disabled = _hasValidationErrors;
     final colorScheme = Theme.of(context).colorScheme;
+    final zulipLocalizations = ZulipLocalizations.of(context);
 
     // Copy FilledButton defaults (_FilledButtonDefaultsM3.backgroundColor)
     final backgroundColor = disabled
@@ -740,7 +770,7 @@ class _SendButtonState extends State<_SendButton> {
         color: backgroundColor,
       ),
       child: IconButton(
-        tooltip: 'Send',
+        tooltip: zulipLocalizations.composeBoxSendTooltip,
 
         // Match the height of the content input. Zeroing the padding lets the
         // constraints take over.
@@ -858,6 +888,7 @@ class _StreamComposeBoxState extends State<_StreamComposeBox> implements Compose
   @override
   Widget build(BuildContext context) {
     final colorScheme = Theme.of(context).colorScheme;
+    final zulipLocalizations = ZulipLocalizations.of(context);
 
     return _ComposeBoxLayout(
       contentController: _contentController,
@@ -865,7 +896,7 @@ class _StreamComposeBoxState extends State<_StreamComposeBox> implements Compose
       topicInput: TextField(
         controller: _topicController,
         style: TextStyle(color: colorScheme.onSurface),
-        decoration: const InputDecoration(hintText: 'Topic'),
+        decoration: InputDecoration(hintText: zulipLocalizations.composeBoxTopicHintText),
       ),
       contentInput: _StreamContentInput(
         narrow: widget.narrow,
diff --git a/lib/widgets/dialog.dart b/lib/widgets/dialog.dart
index 985e7cd5f99..717c18b09b2 100644
--- a/lib/widgets/dialog.dart
+++ b/lib/widgets/dialog.dart
@@ -1,4 +1,5 @@
 import 'package:flutter/material.dart';
+import 'package:flutter_gen/gen_l10n/zulip_localizations.dart';
 
 Widget _dialogActionText(String text) {
   return Text(
@@ -14,12 +15,12 @@ Widget _dialogActionText(String text) {
   );
 }
 
-// TODO(i18n): title, message, and action-button text
 Future<void> showErrorDialog({
   required BuildContext context,
   required String title,
   String? message,
 }) {
+  final zulipLocalizations = ZulipLocalizations.of(context);
   return showDialog(
     context: context,
     builder: (BuildContext context) => AlertDialog(
@@ -28,7 +29,7 @@ Future<void> showErrorDialog({
       actions: [
         TextButton(
           onPressed: () => Navigator.pop(context),
-          child: _dialogActionText('OK')),
+          child: _dialogActionText(zulipLocalizations.dialogButtonOK)),
       ]));
 }
 
@@ -39,6 +40,7 @@ void showSuggestedActionDialog({
   required String? actionButtonText,
   required VoidCallback onActionButtonPress,
 }) {
+  final zulipLocalizations = ZulipLocalizations.of(context);
   showDialog(
     context: context,
     builder: (BuildContext context) => AlertDialog(
@@ -47,9 +49,9 @@ void showSuggestedActionDialog({
       actions: [
         TextButton(
           onPressed: () => Navigator.pop(context),
-          child: _dialogActionText('Cancel')),
+          child: _dialogActionText(zulipLocalizations.dialogButtonCancel)),
         TextButton(
           onPressed: onActionButtonPress,
-          child: _dialogActionText(actionButtonText ?? 'Continue')),
+          child: _dialogActionText(actionButtonText ?? zulipLocalizations.dialogButtonContinue)),
       ]));
 }
diff --git a/lib/widgets/lightbox.dart b/lib/widgets/lightbox.dart
index 829b7ab5270..8a8cb3495a5 100644
--- a/lib/widgets/lightbox.dart
+++ b/lib/widgets/lightbox.dart
@@ -1,5 +1,6 @@
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
+import 'package:flutter_gen/gen_l10n/zulip_localizations.dart';
 import 'package:intl/intl.dart';
 
 import '../api/model/model.dart';
@@ -70,12 +71,13 @@ class _CopyLinkButton extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
+    final zulipLocalizations = ZulipLocalizations.of(context);
     return IconButton(
-      tooltip: 'Copy link',
+      tooltip: zulipLocalizations.lightboxCopyLinkTooltip,
       icon: const Icon(Icons.copy),
       onPressed: () async {
-        // TODO(i18n)
-        copyWithPopup(context: context, successContent: const Text('Link copied'),
+        copyWithPopup(context: context,
+          successContent: Text(zulipLocalizations.lightboxCopyLinkSuccessToast),
           data: ClipboardData(text: url.toString()));
       });
   }
@@ -136,7 +138,7 @@ class _LightboxPageState extends State<_LightboxPage> {
     if (_headerFooterVisible) {
       // TODO(#45): Format with e.g. "Yesterday at 4:47 PM"
       final timestampText = DateFormat
-        .yMMMd(/* TODO(i18n): Pass selected language here, I think? */)
+        .yMMMd(/* TODO(#278): Pass selected language here, I think? */)
         .add_Hms()
         .format(DateTime.fromMillisecondsSinceEpoch(widget.message.timestamp * 1000));
 
diff --git a/lib/widgets/login.dart b/lib/widgets/login.dart
index ec140c04740..c80d06e718f 100644
--- a/lib/widgets/login.dart
+++ b/lib/widgets/login.dart
@@ -1,4 +1,5 @@
 import 'package:flutter/material.dart';
+import 'package:flutter_gen/gen_l10n/zulip_localizations.dart';
 
 import '../api/core.dart';
 import '../api/exception.dart';
@@ -42,17 +43,17 @@ enum ServerUrlValidationError {
     }
   }
 
-  String message() { // TODO(i18n)
+  String translatedMessage(ZulipLocalizations zulipLocalizations) {
     switch (this) {
       case empty:
-        return 'Please enter a URL.';
+        return zulipLocalizations.serverUrlValidationErrorEmpty;
       case invalidUrl:
-        return 'Please enter a valid URL.';
+        return zulipLocalizations.serverUrlValidationErrorInvalidUrl;
       case noUseEmail:
-        return 'Please enter the server URL, not your email.';
+        return zulipLocalizations.serverUrlValidationErrorNoUseEmail;
       case unsupportedSchemeZulip:
       case unsupportedSchemeOther:
-        return 'The server URL must start with http:// or https://.';
+        return zulipLocalizations.serverUrlValidationErrorUnsupportedScheme;
     }
   }
 }
@@ -135,11 +136,13 @@ class _AddAccountPageState extends State<AddAccountPage> {
   }
 
   Future<void> _onSubmitted(BuildContext context) async {
+    final zulipLocalizations = ZulipLocalizations.of(context);
     final url = _parseResult.url;
     final error = _parseResult.error;
     if (error != null) {
       showErrorDialog(context: context,
-        title: 'Invalid input', message: error.message());
+        title: zulipLocalizations.loginInvalidInput,
+        message: error.translatedMessage(zulipLocalizations));
       return;
     }
     assert(url != null);
@@ -158,7 +161,8 @@ class _AddAccountPageState extends State<AddAccountPage> {
         // TODO(#105) give more helpful feedback; see `fetchServerSettings`
         //   in zulip-mobile's src/message/fetchActions.js.
         showErrorDialog(context: context,
-          title: 'Could not connect', message: 'Failed to connect to server:\n$url');
+          title: zulipLocalizations.loginCouldNotConnectTitle,
+          message: zulipLocalizations.loginCouldNotConnectMessage(url.toString()));
         return;
       }
       // https://github.com/dart-lang/linter/issues/4007
@@ -180,13 +184,14 @@ class _AddAccountPageState extends State<AddAccountPage> {
   @override
   Widget build(BuildContext context) {
     assert(!PerAccountStoreWidget.debugExistsOf(context));
+    final zulipLocalizations = ZulipLocalizations.of(context);
     final error = _parseResult.error;
     final errorText = error == null || error.shouldDeferFeedback()
       ? null
-      : error.message();
+      : error.translatedMessage(zulipLocalizations);
 
     return Scaffold(
-      appBar: AppBar(title: const Text('Add an account'),
+      appBar: AppBar(title: Text(zulipLocalizations.loginAddAnAccount),
         bottom: _inProgress
           ? const PreferredSize(preferredSize: Size.fromHeight(4),
               child: LinearProgressIndicator(minHeight: 4)) // 4 restates default
@@ -211,7 +216,7 @@ class _AddAccountPageState extends State<AddAccountPage> {
                   // …but leave out unfocusing the input in case more editing is needed.
                 },
                 decoration: InputDecoration(
-                  labelText: 'Your Zulip server URL',
+                  labelText: zulipLocalizations.loginServerUrlInputLabel,
                   errorText: errorText,
                   helperText: kLayoutPinningHelperText,
                   hintText: 'your-org.zulipchat.com')),
@@ -220,7 +225,7 @@ class _AddAccountPageState extends State<AddAccountPage> {
                 onPressed: !_inProgress && errorText == null
                   ? () => _onSubmitted(context)
                   : null,
-                child: const Text('Continue')),
+                child: Text(zulipLocalizations.dialogButtonContinue)),
             ])))));
   }
 }
@@ -289,10 +294,13 @@ class _PasswordLoginPageState extends State<PasswordLoginPage> {
         // TODO(#105) give more helpful feedback. The RN app is
         //   unhelpful here; we should at least recognize invalid auth errors, and
         //   errors for deactivated user or realm (see zulip-mobile#4571).
+        final zulipLocalizations = ZulipLocalizations.of(context);
         final message = (e is ZulipApiException)
-          ? 'The server said:\n\n${e.message}'
+          ? zulipLocalizations.loginServerErrorMessage(e.toTranslatedString(zulipLocalizations))
           : e.message;
-        showErrorDialog(context: context, title: 'Login failed', message: message);
+        showErrorDialog(context: context,
+          title: zulipLocalizations.loginServerLoginFailed,
+          message: message);
         return;
       }
 
@@ -335,6 +343,7 @@ class _PasswordLoginPageState extends State<PasswordLoginPage> {
   @override
   Widget build(BuildContext context) {
     assert(!PerAccountStoreWidget.debugExistsOf(context));
+    final zulipLocalizations = ZulipLocalizations.of(context);
     final requireEmailFormatUsernames = widget.serverSettings.requireEmailFormatUsernames;
 
     final usernameField = TextFormField(
@@ -350,8 +359,8 @@ class _PasswordLoginPageState extends State<PasswordLoginPage> {
       validator: (value) {
         if (value == null || value.trim().isEmpty) {
           return requireEmailFormatUsernames
-            ? 'Please enter your email.'
-            : 'Please enter your username.';
+            ? zulipLocalizations.loginValidationRequireEmail
+            : zulipLocalizations.loginValidationRequireUsername;
         }
         if (requireEmailFormatUsernames) {
           // TODO(#106): validate is in the shape of an email
@@ -360,7 +369,9 @@ class _PasswordLoginPageState extends State<PasswordLoginPage> {
       },
       textInputAction: TextInputAction.next,
       decoration: InputDecoration(
-        labelText: requireEmailFormatUsernames ? 'Email address' : 'Username',
+        labelText: requireEmailFormatUsernames
+          ? zulipLocalizations.loginValidationRequireEmailLabel
+          : zulipLocalizations.loginValidationRequireUsernameLabel,
         helperText: kLayoutPinningHelperText,
       ));
 
@@ -372,14 +383,14 @@ class _PasswordLoginPageState extends State<PasswordLoginPage> {
       autovalidateMode: AutovalidateMode.onUserInteraction,
       validator: (value) {
         if (value == null || value.isEmpty) {
-          return 'Please enter your password.';
+          return zulipLocalizations.loginValidationPassword;
         }
         return null;
       },
       textInputAction: TextInputAction.go,
       onFieldSubmitted: (value) => _submit(),
       decoration: InputDecoration(
-        labelText: 'Password',
+        labelText: zulipLocalizations.loginValidationPasswordLabel,
         helperText: kLayoutPinningHelperText,
         // TODO(material-3): Simplify away `Semantics` by using IconButton's
         //   M3-only params `isSelected` / `selectedIcon`, after fixing
@@ -389,14 +400,14 @@ class _PasswordLoginPageState extends State<PasswordLoginPage> {
         //   [ButtonStyleButton].)
         suffixIcon: Semantics(toggled: _obscurePassword,
           child: IconButton(
-            tooltip: 'Hide password',
+            tooltip: zulipLocalizations.loginHidePassword,
             onPressed: _handlePasswordVisibilityPress,
             icon: _obscurePassword
               ? const Icon(Icons.visibility_off)
               : const Icon(Icons.visibility)))));
 
     return Scaffold(
-      appBar: AppBar(title: const Text('Log in'),
+      appBar: AppBar(title: Text(zulipLocalizations.loginPageTitle),
         bottom: _inProgress
           ? const PreferredSize(preferredSize: Size.fromHeight(4),
               child: LinearProgressIndicator(minHeight: 4)) // 4 restates default
@@ -416,7 +427,7 @@ class _PasswordLoginPageState extends State<PasswordLoginPage> {
                   const SizedBox(height: 8),
                   ElevatedButton(
                     onPressed: _inProgress ? null : _submit,
-                    child: const Text('Log in')),
+                    child: Text(zulipLocalizations.loginFormSubmitLabel)),
                 ])))))));
   }
 }
diff --git a/lib/widgets/message_list.dart b/lib/widgets/message_list.dart
index 79fdd24cfda..8bf20d5a561 100644
--- a/lib/widgets/message_list.dart
+++ b/lib/widgets/message_list.dart
@@ -536,7 +536,7 @@ final _kRecipientHeaderDateStyle = TextStyle(
   color: const HSLColor.fromAHSL(0.75, 0, 0, 0.15).toColor(),
 );
 
-final _kRecipientHeaderDateFormat = DateFormat('y-MM-dd', 'en_US'); // TODO(i18n)
+final _kRecipientHeaderDateFormat = DateFormat('y-MM-dd', 'en_US'); // TODO(#278)
 
 /// A widget with the distinctive chevron-tailed shape in Zulip recipient headers.
 class RecipientHeaderChevronContainer extends StatelessWidget {
diff --git a/test/api/core_test.dart b/test/api/core_test.dart
index 9b1cc26b619..a2e469eaf5d 100644
--- a/test/api/core_test.dart
+++ b/test/api/core_test.dart
@@ -3,6 +3,7 @@ import 'dart:io';
 
 import 'package:checks/checks.dart';
 import 'package:checks/context.dart';
+import 'package:flutter_gen/gen_l10n/zulip_localizations.dart';
 import 'package:http/http.dart' as http;
 import 'package:test/scaffolding.dart';
 import 'package:zulip/api/core.dart';
@@ -155,7 +156,11 @@ void main() {
 
     checkRequest(http.ClientException('Oops'), it()..message.equals('Oops'));
     checkRequest(const TlsException('Oops'), it()..message.equals('Oops'));
-    checkRequest((foo: 'bar'), it()..message.equals('Network request failed'));
+    final zulipLocalizations = lookupZulipLocalizations(ZulipLocalizations.supportedLocales.first);
+    final expectedMessage = zulipLocalizations.apiConnectionNetworkRequestFailed;
+    checkRequest((foo: 'bar'), it()
+      ..toTranslatedString(zulipLocalizations)
+      .equals(expectedMessage));
   });
 
   test('API 4xx errors, well formed', () async {
diff --git a/test/api/exception_checks.dart b/test/api/exception_checks.dart
index 60b792fda22..5e2f5458c01 100644
--- a/test/api/exception_checks.dart
+++ b/test/api/exception_checks.dart
@@ -1,9 +1,12 @@
 import 'package:checks/checks.dart';
+import 'package:flutter_gen/gen_l10n/zulip_localizations.dart';
 import 'package:zulip/api/exception.dart';
 
 extension ApiRequestExceptionChecks on Subject<ApiRequestException> {
   Subject<String> get routeName => has((e) => e.routeName, 'routeName');
-  Subject<String> get message => has((e) => e.message, 'message');
+  Subject<String?> get message => has((e) => e.message, 'message');
+  Subject<String> toTranslatedString(ZulipLocalizations zulipLocalizations) =>
+    has((e) => e.toTranslatedString(zulipLocalizations), 'getTranslatedMessage');
 }
 
 extension ZulipApiExceptionChecks on Subject<ZulipApiException> {
diff --git a/test/widgets/action_sheet_test.dart b/test/widgets/action_sheet_test.dart
index 0d143ae5e3f..64c4c2cd5b8 100644
--- a/test/widgets/action_sheet_test.dart
+++ b/test/widgets/action_sheet_test.dart
@@ -1,6 +1,7 @@
 import 'package:checks/checks.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
+import 'package:flutter_gen/gen_l10n/zulip_localizations.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:zulip/api/model/model.dart';
 import 'package:zulip/api/route/messages.dart';
@@ -48,6 +49,8 @@ Future<void> setupToMessageActionSheet(WidgetTester tester, {
 
   await tester.pumpWidget(
     MaterialApp(
+      localizationsDelegates: ZulipLocalizations.localizationsDelegates,
+      supportedLocales: ZulipLocalizations.supportedLocales,
       home: GlobalStoreWidget(
         child: PerAccountStoreWidget(
           accountId: eg.selfAccount.id,
diff --git a/test/widgets/autocomplete_test.dart b/test/widgets/autocomplete_test.dart
index 164cd765155..6f540111ca9 100644
--- a/test/widgets/autocomplete_test.dart
+++ b/test/widgets/autocomplete_test.dart
@@ -1,5 +1,6 @@
 import 'package:checks/checks.dart';
 import 'package:flutter/material.dart';
+import 'package:flutter_gen/gen_l10n/zulip_localizations.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:zulip/api/model/model.dart';
 import 'package:zulip/api/route/messages.dart';
@@ -40,6 +41,8 @@ Future<Finder> setupToComposeInput(WidgetTester tester, {
 
   await tester.pumpWidget(
     MaterialApp(
+      localizationsDelegates: ZulipLocalizations.localizationsDelegates,
+      supportedLocales: ZulipLocalizations.supportedLocales,
       home: GlobalStoreWidget(
         child: PerAccountStoreWidget(
           accountId: eg.selfAccount.id,
diff --git a/test/widgets/clipboard_test.dart b/test/widgets/clipboard_test.dart
index ace8c4d8311..2b06625377a 100644
--- a/test/widgets/clipboard_test.dart
+++ b/test/widgets/clipboard_test.dart
@@ -32,7 +32,6 @@ void main() {
             body: Builder(builder: (context) => Center(
               child: ElevatedButton(
                 onPressed: () async {
-                  // TODO(i18n)
                   copyWithPopup(context: context, successContent: const Text('Text copied'),
                     data: ClipboardData(text: text));
                 },
diff --git a/test/widgets/content_test.dart b/test/widgets/content_test.dart
index 8078d16ab7c..cd9324ebe94 100644
--- a/test/widgets/content_test.dart
+++ b/test/widgets/content_test.dart
@@ -3,6 +3,7 @@ import 'dart:io';
 import 'package:checks/checks.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
+import 'package:flutter_gen/gen_l10n/zulip_localizations.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:url_launcher/url_launcher.dart';
 import 'package:zulip/api/core.dart';
@@ -68,6 +69,8 @@ void main() {
       addTearDown(testBinding.reset);
 
       await tester.pumpWidget(GlobalStoreWidget(child: MaterialApp(
+        localizationsDelegates: ZulipLocalizations.localizationsDelegates,
+        supportedLocales: ZulipLocalizations.supportedLocales,
         home: PerAccountStoreWidget(accountId: eg.selfAccount.id,
           child: BlockContentList(
             nodes: parseContent(html).nodes)))));