I know about this question: Can Stack Exchange employees understand French?
But I think the tags are more important than the message text, so I created a new question.
Tags are used in many ways, like finding the questions, finding similar questions (by SE engine), etc. I tagged my own questions in English, but I saw many questions are tagged using French words, which is expected in this community. But having two sets of overlapping tags is not very good, I think.
So, either StackExchange should have a way to equalize tags in different languages, or there should be rule about the language used in tags, (like only use French, or use both English and French) and that rule, in my opinion, should appear on the question authoring page, beside the tags text box.