I am sure you could just edit those files directly, but I don't know what you want to change... the drop down in the CMS or the stuff on the live site? If it is on the live site there is an awesome example in the the book that shows flags instead... I use that (it needed a smal hack but works a treat).
Thanks Barry your a star, I actually have the book, but haven't got round to reading it yet, silly me I should have looked in there first.
Yes I wanted the change the names on the live site, instead of English (US) and Welsh I want to have English and Cymraeg.