Related to the question: Une Personne Morale... Is it a practical and logical appellation? and the comments therein.
Should a native speaker of French post his or her questions in French? The OP said that he or she was mistaken because the title of the forum is not written in French, and neither are some of the questions.
The discussion linked by jlliagre in a comment first took place 9 years ago, at the very beginning of this site, and it seems like the biggest preference was for French as the main language on this site. While I do agree that as a learner, forcing yourself to speak the language you're learning helps you making progress faster, I do not agree with setting this as a rule, native or not.
Personally, I want this site to be as welcoming as possible for learners. I have witnessed it around me: not everyone learns the same way, and some people are not confident enough (or their level is not advanced enough) to ask a question in French. In the same spirit, it seems well-intentioned to ask some questions in English so that potential learners who stumble upon them are able to understand them and make progress.
Also, I'm a native French speaker and yet the only time I ever asked a question (on Meta) was in English, because it felt more natural on SE and also more inclusive. Maybe that's how the OP felt on the post you linked.
So my opinion is that, native or not, we let people use the language they are the most comfortable with at a given time, maybe we encourage them towards French in a nice way like you did, and everything should be fine! :)
There is no obligation to do so. As StackExchange is primarily in English all the GUI is in English but as the site is about French language we can use French.
I can understand the OP as to use FL you see all the GUI in English.
The bottom line is that the asker should ask in the language that would be easier for him to ask and get understood. (English or French)
I would add that for French learners often an English question is easier to find if they search for a topic.
The only sites whose GUI language StackExchange changed are these 3; but on these sites you are required to use the correct language.
Ideally, the response should use the OP's language.
French language is the topic, and can be discussed in any language. If SE opens a forum on German litterature, it doesn't mean users should interact in German on that forum :)
