l10n: Update translations from Weblate!

Fixes most of #276.  (The remaining work is to have this process
a bit more automated; that's in progress.)  Thanks to Alex for
doing the bulk of the actual work of setting this up.

The steps to produce this were those in the current version
of .github/workflows/update-translations.yml :

    git remote add weblate https://hosted.weblate.org/git/zulip/zulip-flutter/
    git fetch weblate
    git merge --ff-only weblate/main

plus:

    tools/check l10n --fix
    git add lib/generated/l10n/
    git commit --amend

Trying the app out, it seems to work great.  If I switch my system
language setting to Polish, I see the new Polish translations.

Co-authored-by: Alex Vandiver <[email protected]>

assets/l10n/app_fr.arb

@@ -0,0 +1 @@
+{}

assets/l10n/app_ja.arb

@@ -1,15 +1,20 @@
 {
   "chooseAccountPageTitle": "アカウントを選択",
+  "@chooseAccountPageTitle": {},
   "chooseAccountButtonAddAnAccount": "新しいアカウントを追加",
+  "@chooseAccountButtonAddAnAccount": {},
   "profileButtonSendDirectMessage": "ダイレクトメッセージを送信",
-  "cameraAccessDeniedTitle": "権限が必要です",
-  "cameraAccessDeniedMessage": "画像をアップロードするには、「設定」で Zulip に追加の権限を許可してください。",
-  "cameraAccessDeniedButtonText": "設定を開く",
-  "subscribedToNChannels": "{num, plural, other{{num}つのチャンネルをフォローしています}}",
+  "@profileButtonSendDirectMessage": {},
   "userRoleOwner": "オーナー",
+  "@userRoleOwner": {},
   "userRoleAdministrator": "管理者",
+  "@userRoleAdministrator": {},
   "userRoleModerator": "モデレータ",
+  "@userRoleModerator": {},
   "userRoleMember": "メンバー",
+  "@userRoleMember": {},
   "userRoleGuest": "ゲスト",
-  "userRoleUnknown": "不明"
+  "@userRoleGuest": {},
+  "userRoleUnknown": "不明",
+  "@userRoleUnknown": {}
 }