Is this diplome valid if i go to Thailand -- to join my girlfriend?

I can't read Dutch but what are you looking for. Do you plan to find a job in Thailand? If so, will this school teach you more than a Thai student studying the same course in Thailand. If not why would they hire you over a Thai? Can you speak, read, write Thai. Unless you have skills that a Thai wouldn't have, there is no reason for someone to hire you. Planning to open your own business here may be a better idea then trying to find work in your field here.