Can I ask here some questions about popular TV-programmes from Belgium?
Of course you can. You can ask about any film or TV-show you like. Well, nearly, there are some limitations as to the shows we deem on-topic here, as well as the kinds of questions we regard as valid, the details of which can be found in the respective section of the help center as well as enquired by discussion here on Movies & TV Meta if necessary. However, being from Belgium or wherever country is not a reason for a show to be off-topic here. The site's language is English, but this doesn't make the language of the site's topic necessarily English.
Do I need special tags to let it know?
Yes, in general questions are to be tagged with the specific movie or TV-show they're about (if they are). It might be, though, that you can't create the tag yourself right away if it doesn't exist yet, due to your low reputation. However, more experienced users will usually chime in to do so. If that doesn't happen, feel free to leave a comment or flag it for moderator attention.
Can I write the details in Dutch and the most important info in English?
This won't go over so well, since the site's language for all intents and purposes is English. While some users might be able (and maybe even willing) to translate non-English parts for you, this isn't in any way required and a question that is not written in English is unfortunately not a valid question and liable to be closed. Neither will you be able to make much sense out of any answers, since they will always have to be in English, too.
Can I make specific tags for a program like "F.C. De Kampioenen", "Bevergem", "Thuis", "Familie" etc and channels like "één", "Canvas", "VTM" etc?
As said, once there is a question about it, you can make a tag for it. But you need the necessary reputation for doing so. However, we don't make tags for specific TV channels here, only for TV-shows and films (and other more general things).
Are popular movies also included?
Sure, even "unpopular" ones. ;-) It's Movies & TV afterall.