The interview isn't mainly in French.
I'm not there yet, but I've been through the application process, so I can help you.
You'll have at least 2 interviews. Are you thinking of French sole or French and linguistics?
The tutors will begin by asking you a couple of settling-down questions. Then, they'll ask you to discuss a poem or a short prose extract (I had a poem) you'll have been given twenty minutes before the interview. They might ask you about a whole host of things, including things as vague as what it's about and what (if any) message the author/poet was getting across. Then, they tend to delve deeper into it. What they're trying to assess is the way you think, so there won't be a right or wrong answer, but they *will* disagree with you a lot; don't worry, they want you to defend your standpoint, but sometimes you have to know when to concede a point.
They'll ask you to read out whatever you've been given, to see how well you pronounce words, what your accent's like and how well you cope with reading unfamiliar stuff.
Then they'll give you an opportunity to speak in the target language. For me, they asked me about what reading I'd done in French, and we proceeded to discuss the finer points of Les Mains Sales and L'Etranger in French. They might ask you what topics in the course of your French learning you've enjoyed studying. Alternatively, they might ask you about the witten work you've submitted.
They might ask you about your personal statement and your interests in francophone culture.
Languages are highly respected degrees, and recognised as one of the most difficult and the broadest. Especially at oxford, the course is very very literature-based (I'm grappling with my first year reading list before I get there, and there are 19 titles between french and german!)
You learn about the literature, politics, philosophy, geography, culture and history, as well as development of language and practical work. You can look at linguistic issues and other related fields. If you do sole French, you can take papers in French Thought and French Cinema.
The success rate for MFL is approximately one in three.
Edit: with regards to what it's like to study there, I don't yet know, but Oxford has Britain's largest French department and the use of a very central building - the Taylor Institute. This is the largest institute in Britain dedicated solely to languages and the same goes for the accompanying Taylorian library.