You can ask in either English or the native language, according to your convenience.
The sites are for both learners and experienced people. So, one can ask questions in either English or in the native language of the site, according to the convenience.
For example, if I am a learner and have a doubt about the gender of nouns in French, I can definitely not ask in French, so I would very much prefer to ask in English, with a short note, which says that I would appreciate an answer in English (however it is the answerer's choice whether they want to answer in English or not).
Native people asking doubts about the language, can ask them in the native language, and can also expect answers in the native language.
But, as Gilles rightly pointed out hereGilles rightly pointed out here, it would be better if the questions are asked in the native language, rather than in English, for the reasons given in his answer.
So, IMHO putting a restriction on the language of the question isn't a healthy decision moving forward.