I would like to offer language options for my members but for those who post in different languages, it would be converted / posted in English. Is this possible? I've honestly never tried anything with the language packs yet so I just wish to confirm how it works.

Language packs just convert vB Phrases to a different language. Like the "User CP" link would be in a different language or the "Search" link but it doesn't change the contents of posts.

Would amost be impossible to code that :p

Translating simple words yes, but translating complete sentences - lol

The problem would be more, especially on a forum.

That everything has to be written in perfect language.