Board Thread:New on the Wiki, FAQ & Help/@comment-1789834-20151021012831/@comment-15636815-20151102203807

I don't have an answer for how to create global (i.e. for all Wikia) translations for the user interface.

You can control these items locally (within only one wikia) on the  page. Just type that page name into the search bar of one of your Welsh wikis, find the message that contains the English content you want to eliminate, and enter the Welsh translation in its place.

That list is REALLY long, though. It's much easier to find what you need if you know the name of the message(s) you need. Even knowing the prefix will help. Alas, I'm fairly new at the admin thing, and I have no information or advice on where to find the messages you seek. The folk on Community Central's Getting Technical forum may be able to help with that part.