That's interesting. I think your decision makes sense, since your french users all speak English and you have much more content in English. In fact it personally irks me when I see reddit's UI in French (for example when I'm not logged in) because I'd rather have the whole site in English than parts of it in French.

In any case I think that if you really wanted to target the French-speaking population, it would be best to have a separate site or at least a clearly separated part of the site.