It really depends on the resources you have available and how much time you have for the study. If this is, for example, a PhD thesis, you will certainly have time, if you plan well.
However, if you want to compare two languages, it's good to plan it well and not arbitrarily choose two languages because you know them. Instead, choose two languages that are similar or different in a particular way, or find such a similarity/difference for French and Chinese, but make sure there's a specific motivation behind that choice, not just convenience.
(My comment above was that you might want to only look at a less studied language. For that matter, even Chinese has been studied quite a bit. My comment was much more general than your question, though-- this is a general problem in the field that most research revolves around these most commonly studied languages so we really don't know very much about the bigger picture. So as a simple solution, if you want an unexplored area, look at second language learning of Swahili by Arabic speakers, or some other combination like that. It may not be practical for you, of course, which is why most research is about English and other European languages. You won't be alone in that.)
