First of all welcome! Where in the Schweiz are you?
As for your questions: 1. many of us here are not Scottish, but still wear the kilt. That said, you will find some people (generally not scottish) who believe only Scots can wear kilts. Don't let a few people stop you. Most people are supportive of kilt wearing whether you are Belgian or Bangladeshi. 2. There are a number of so-called universal tartans. The most common of these is the Black Watch. That is if you do not feel comfortable wearing a clan tartan. Opinions on this vary. I have heard that some people do not like others wearing their tartans just because. But there is absolutely no rule against it. Most people on this forum seem to feel that if you know something about the tartan you are wearing and you wear it respectfully then it is ok. Please be aware that this is not everyones opinion, but there's no law at all to say you can't wear any tartan you want.
3. Where to buy? A traditional kilt is going to be expensive. No way around it. You can search for used ones, or ones a kilt maker is trying to sell. I was lucky, in that my kilt was a traditional one made for someone my size and he never picked it up, so I got a deal. Maybe you'll have some luck too! There are a number of kilt-makers/sponsers on this forum. Search the reviews and ask questions and you'll find out what's best for you and your budget. Good luck!
As for a Beligan tartan. I searched the Scottish Tartans Authority website and didn't find one. But maybe you could research Scottish families. I believe some of them had roots in Belgium. Perhaps you could find a family that came from your area of Belgium all those years ago, and wear their tartan with pride.
from South Wales UK! 
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