UX: Fallback locale warning improvements. (#11775)

- Improve warning message.
- Only display the warning if the language has a fallback and either "allow_user_locale", or "set_locale_from_accept_language_header" are enabled.

app/assets/javascripts/admin/addon/controllers/admin-site-text-index.js

@@ -42,6 +42,15 @@ export default Controller.extend({
     }
   },
 
+  @discourseComputed("locale")
+  showFallbackLocaleWarning() {
+    return (
+      (this.siteSettings.allow_user_locale ||
+        this.siteSettings.set_locale_from_accept_language_header) &&
+      this.fallbackLocaleFullName
+    );
+  },
+
   actions: {
     edit(siteText) {
       this.transitionToRoute("adminSiteText.edit", siteText.get("id"), {