The biggest difference between the SAT and ACT is the depth and time of the material. The SAT tests three (now two) sections, Math and Reading, while the ACT tests 4 (or 5), English, Math, Reading, Science, and an optional Writing section (which you really should take, otherwise it makes you look lazy). The SAT gives you more time, but the wording can be confusing. The ACT gives you less time, but most questions are pretty straightforward.
You can't directly show your proficiency in French on the SAT or ACT, but you can take an SAT Subject Test. An SAT Subject Test evaluates your ability on a single topic, such as French, Spanish, Physics, etc. The French Subject Test consists of 85 multiple choice questions and lasts for about an hour. It's supposed to be really easy for native speakers (not sure if you are or not). You can take a SAT Subject Test regardless of whether you take the SAT or ACT.