Funny story about that. We (reddit) had a long debate about what the default UI and content should be back when we internationalized (2008 I think?). Should we use the Accept-Language header to select both the UI and the content, or just the UI? We started by making the assumption that if your preferred language was French, for example, you'd want French content.
We got a ton of feedback from French speakers that they preferred the English content, so we ended up settling on selecting a UI based on the A-L header but giving the default English content (unless you specifically set it otherwise).
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.
If it's user-created content, then it has a serious network effect - for the smaller communities, they will often prefer larger(=~better) english content to a smaller pool of their local content.
It would be a different story if the content available was comparable, but it usually isn't.
We got a ton of feedback from French speakers that they preferred the English content, so we ended up settling on selecting a UI based on the A-L header but giving the default English content (unless you specifically set it otherwise).