A lot of the roms I have are in japanese, is it possible to switch it to the english version?

Some games support it, some games don't. Which ones are you looking at in particular?

If it did allow it, is there a botton somewhere on MAME for that?

Not a button. A MAME in-game menu entry called DIP Switches/Settings (I forget the actual name). Not all games that use DIP switches have region switches in them. If you have a japanese game and it does not have region DIP switches, you need to find of non-japanese romset of that game.

I see, I'll look at the dip switches. So for the games that you can change the region, that means that for that game there is no such thing as US or JP version since all languages are supported right?

Not entirely true. Some games, such as King of Gladiator (a Neo-Geo bootleg), has a "jumper" on the circuit board itself that sets the region. It's permanently (normally) set to Japanese or English on a real board.

I know of other games that have eeproms [non volatile ram (ram that doesn't erase itself once the power is off)] that permanently set the region. If you swap out the eeproms, you can change the region. ^^

Some Japan-only games still have English language programmed into them. PGM & CPS2 games, for example, have several languages programmed into them. Change a single byte of the program rom, and the language changes.