With this extension you can easily complete poor language add-ons, or create a new one from scratch.
You will find it in Extensions -> Modules -> Translate.
First import all the translations to the database, this may take a while.
This extension is using the Event system instead of OCMOD, with lower priority than the built-in Language Editor (-1 vs. 0).
Should work on all 3.x but not tested.

Please share your experiences in a comment.

Have fun! :)

cheers

My other extensions:
Core SEO URL
Disable Routes