add inital data for language selector (#20620)

* add inital data for language selector

* Update compute-initial-ha-form-data.ts

src/components/ha-form/compute-initial-ha-form-data.ts

@@ -71,6 +71,10 @@ export const computeInitialHaFormData = (
         if (selector.country?.countries?.length) {
           data[field.name] = selector.country.countries[0];
         }
+      } else if ("language" in selector) {
+        if (selector.language?.languages?.length) {
+          data[field.name] = selector.language.languages[0];
+        }
       } else if ("duration" in selector) {
         data[field.name] = {
           hours: 0,
@@ -93,7 +97,9 @@ export const computeInitialHaFormData = (
       ) {
         data[field.name] = {};
       } else {
-        throw new Error("Selector not supported in initial form data");
+        throw new Error(
+          `Selector ${Object.keys(selector)[0]} not supported in initial form data`
+        );
       }
     }
   });