I have users from Mexico, UK and USA. I wanted to have a option for any users to switch the language if the forum, and it would change english into any language I choose.
You can't simply "switch" the language. Although, provided that you have the language pack for the languages, you can allow users to choose the language they want. This does not apply to posts though - as they cannot be easily translated.
So there is no modification to change the language of posts??
Look at how bad the online translators are - I doubt it will turn posts into something properly understandable (if you could "integrate" them), given the amount of typos common.