You should be able to post your question in French, if you can’t state it clearly in English. Just because a forum’s preferred language is English does not mean that you can only use English here.
However, if you don’t use English, your question may not receive the wide attention it might warrant. Also, you should be prepared to get an English answer even when the original question is posed in French. Sometimes the responder may re-state your French question in English first, so that those of us who don’t understand the original language can take part in the discussion.
Matt, who is most active around here, is actually French, and I have noticed him reply in French on a few occasions.