PLT-5549: No feedback when trying to save localization settings in System Console when Available Languages doesn't include the default client language (#7554)

* Added server-side validation check for localization settings. Refactored validation checks to be easier to read

* Fixed gofmt issues

i18n/en.json

@@ -4383,6 +4383,10 @@
     "id": "model.config.is_valid.data_retention.message_retention_days_too_low.app_error",
     "translation": "Message retention must be one day or longer."
   },
+  {
+    "id": "model.config.is_valid.localization.available_locales.app_error",
+    "translation": "Available Languages must contain Default Client Language"
+  },
   {
     "id": "model.config.is_valid.elastic_search.aggregate_posts_after_days.app_error",
     "translation": "Elasticsearch AggregatePostsAfterDays setting must be a number greater than or equal to 1"