The start-up scene is currently definitely at an interesting point: really young and active (people already shared links to Startup studios, you should have a look at them). The meetup scene is great too.
The salary is still a pity though (compared to the States, Switzerland or Baltic countries). For a middle developer position, you may be around 40K euros, which is "nice in France", but not a lot, especially after paying your taxes.
> How good does my French need to be?
To work, English is fine, to live, it's another problem. French people are usually really kind to people trying to learn French as it's a hard language. If you don't care about french, people won't care about you.
> Are there any languages/platforms/etc. that are popular there more than in Silicon Valley/Fairfax County
There's a bit of everything, but the Web and Robotics are especially important.