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Xurumo:
There's no particular difficulty in doing this. You should have a folder named with the new language and containing the files with the necessary language constants. Copy that folder under the languages folder in your XOOPS root directory. That'll be enough to show in your admin side the language in your language selector; that'll also be enough for the global and general constants to appear in your new language.
You have to make sure that the modules also have, in their own language folder, a folder for your new language with the necessary files. Otherwise, you'll have the module's strings in english and the rest in your language.
Cheers.