I think it is only sometimes good advice, and should be left out of the help. As it says, there's no obligation, and both French and English are accepted. The help should be about asking good questions, not telling people how to learn.
Speaking as very much a beginner, I feel this is likely unhelpful advice for the person posting the question. While I do appreciate the notion of learning by doing, writing in a foreign language by using a dictionary for most of the words is a very arduous process.
Further, it's been fairly well established that answers are preferred to be in the language of the question. Someone learning the language may be able to, with effort and subsequent editing from the community, ask a simple question in French, but then they'll have to decipher answers in French, likely written at a higher level than the question.
Both writing the question and reading the answers can of course be learning processes, as suggested, but particularly for beginners, they are unlikely to be efficient or productive learning processes. I know that if I have a basic specific question, the thing that will help me most is a quick, understandable answer and returning to my lessons. But the help seems to be trying to tell me that I'd learn more if I asked in French.
If on the other hand, I were comfortable enough to ask and read answers in French, it'd be a great idea - no argument there. But at that point, I wouldn't need the help page to tell me so either. It's great to encourage people to learn, but I don't think the help needs to be quite so prescriptive.