Well, I am ambivalent in this issue. I have already suggested a way to at least filter out your search results to those languages you can understand. That would keep the search for new topics and the likes at least useable. There are ups and downs to being able to ask questions in your own language. On the plus side, it is often easier to express yourself, and that makes formulating a good and precise question easier. Good, well formulated questions tend to get much better answers than questions that are hard to parse. On the other hand you probably limit your audience, as less people are able to read it. So, it is something to considder on a per-user basis. Personally, I prefer to ask my questions in English, as I am fairly confident that I am able to write in that language in a way that gets my question across well enough. However, I also understand that not everyone is able to do that, and I would be willing to help somebody in Dutch, for instance.
If your command of English is of such a level that you can express your question well, then I think it makes sense to ask it in English, especially if the user community in your language is not so big. However, don't underestimate the number of users in some languages. There have been French and Russian language Qt forums or lists for years, and probably others that I am not aware of. The number of users speaking Spanish, Portugese or Chinese also probably should not be underestimated.