I really enjoy French (WJEC) and would like to take it for a level but I'm worried it'll be ridiculously hard. I'd like to get all As at AS so I need to pick something that I enjoy but also something that will allow me to get an A.
The easiest way to get an A at AS level is to pick subjects you enjoy, are passionate about and are good at. Usually the last one comes with the first two
What is your current level of ability? If you already have quite a high level of French then it will be easier for you to continue to do well at A level, and I also agree that if you really do enjoy it, that will make it so much easier for you as learning and revision won't be as much of a chore! Also consider whether it will be helpful/relevant in the future.