Since you have initiated the first move and been given an answer you can not back out anymore. Since you say that the nicer of the bosses has offered to speak with you about the problem then you should be feeling positive - at least it's not one of the not so nice bosses
But you should prepare your evidence for this meeting and certainly do not speak aggressively. State the facts plainly and make sure you tell him how you've acted to try to resolve the problems as he will probably ask you if you have tried speaking directly with the colleagues and if not, why not. It's important to demonstrate you are doing all you can to maintain a good working relationship.
I think that people in Switzerland generally try to resolve these kind of matters in an informal way before going all legal.
Some suggestions to the OP:
Buy a good voice recorder (or use your Handy) and start recording "everything".
Make printouts of emails that show the intent to bully.
If everyone is against you... I hate to say this but I would personally start looking for another job because that's a no-win situation and will only end in a mental breakdown