The Uni is good, and as far as languages go the EUROS is top notch. I've had no experience with the English dept, but I seem to hear good things about them. As for French, it's only across the hall from my Spanish tutor and downstairs from my Italian ones, and all my coursemates seem to be doing it and they're enjoying it and progressing.
We have an amazing ERASMUS society here, so you can always make friends with French people to help you along (in fact, the French people I've met this year have been the nicest girls ever, all of them from Nantes and so enthusiastic and happy to talk to you). You get your choice of a third subject in first year, so you can branch out and try your hand at linguistics, or another language, or even something totally different like Philosophy or Archaeology.
It shouldn't matter that it's "low" on the leagues for a Russell Uni. That's probably based on things like "Staff -> Students" or "Graduate Prospects" rather than quality of teaching, which is the most important part. I've got friends in Exeter doing both English Lit and French, and while they seem to be enjoying it and getting a lot out of their degree (one is doing a creative writing module; the other one has picked up Portuguese on the side), I can't stress enough how Cardiff is:
a) brilliant for the way it teaches
b) linked with so many good Unis on the continent (should you continue with your French), ranging from Montpellier and Grenobles, to Nantes and Limoges, and even Paris and the Instituts d'études politiques at Bordeaux.
c) socially vibrant - from the ERASMUS society and the associated languages socs, to other societies and the AU. (Admittedly, if you're into dance, Cardiff's dancesport pales in comparison to Exeter's - my ex is the President of the latter as of next year and she'll no doubt improve standards further...)
d) a city that doesn't exactly sleep. I've spent the two hours writing up a Rough Guide to Cardiff on my blog, and it's nowhere near a complete overview, but, for the daytime, nighttime, drinktime, and clubtime, it's a brilliant city - Exeter does have its gems (Timepiece not being amongst them; though nowhere tops The Old Firehouse there, just for charm value).
I suppose that's all the help I can give you. No clue on Leeds, but, Exeter v Cardiff...the 'diff wins hands down, despite the soft spot I have for Exeter.