Thread:Project:Support desk/wiki text translator for MW 1.16/reply

Extension:Translate allows you to create translations of pages (I believe, I've never used it, but its in use in several high profile wikis).

If you're looking for automatic translation, there's things like Extension:Live_Translate (I've also never used it).

-

There are certain hacks to make pages translated based on the language in special:preferences without any extensions. Specificly see int: in help:Magic Words. Using such facilities are quite hacky/ugly, but they are used succesfully to translate templates at Wikimedia Commons