i18n: pass only the ISO code string to 'translate()'

Don't use the whole Hash everywhere.
Also fall back nicely to english string if no translation exists.

src/invidious/videos.cr

@@ -275,7 +275,7 @@ struct Video
     end
   end
 
-  def to_json(locale : Hash(String, JSON::Any) | Nil, json : JSON::Builder)
+  def to_json(locale : String?, json : JSON::Builder)
     json.object do
       json.field "type", "video"
 
@@ -475,7 +475,7 @@ struct Video
   end
 
   # TODO: remove the locale and follow the crystal convention
-  def to_json(locale : Hash(String, JSON::Any) | Nil, _json : Nil)
+  def to_json(locale : String?, _json : Nil)
     JSON.build { |json| to_json(locale, json) }
   end