Cleanup unused i18n strings (#23426)

* Remove partial 422/500 error translations

* Add missing siblings for i18n unused warnings

* Enable i18n unused string checking for all locales

config/locales/ga.yml

@@ -369,9 +369,7 @@ ga:
     '404': The page you are looking for isn't here.
     '406': This page is not available in the requested format.
     '410': The page you were looking for doesn't exist here anymore.
-    '422': 
     '429': Too many requests
-    '500': 
     '503': The page could not be served due to a temporary server failure.
   exports:
     archive_takeout:
@@ -437,6 +435,8 @@ ga:
     prev: Ceann roimhe seo
   preferences:
     other: Eile
+    posting_defaults: Posting defaults
+    public_timelines: Public timelines
   relationships:
     follow_selected_followers: Lean leantóirí roghnaithe
     followers: Leantóirí