At first the problem is your false expectations. You've agreed to be expat, which means you count with not being able to use your native language at work, etc. You assume, that only because the company is international, the same would be expected from local workers.
It isn't so. The official language of the company may be English, which means that official communication will be in English, and reasonable level of fluency in English will be required by recruiting (which doesn't imply fluency compared to mother language).
Your collegues are communicating unoficially, because they are talking between themselves, and not on official meetings. If they would use the language you don't know and are not expected to know (Flemish) in communication including you (for example, speaking about requirements in project, work to be done) it would be something other. But the fact that you are sitting in the same office doesn't mean, that any communication that happens within that office has something to do with you. They may be talking about specific problems they had, and their fluency in English may be not enough to discuss them freely in that language.
Forbidding of using of any language, especially the language of given country, is no go, and doing so could be a serious violation of labour law, as well as citizen rights! Don't even try to go that way!
The problem is primarily structural, because you are working for international company, and you were right to expect you will be working with many foreigners, but instead you've landed in the office where you are the only foreigner, what's worse with poor knowledge of local language. What can you do is to talk with management about that problem. You can argue that it's counterproductive if you have to work with people, with which you can't communicate freely, because you aren't good enough in Flemish, and they are not fluent enough in English.
For sure it can be reorganized so, that you will be placed in team, where there are enough foreigners, so that the communication will automatically happen in English, and the "locals" speaking Flemish would be only an exception.