As some others mentioned, France is not the best when it comes to tech salaries. That being said, if you want to make more money, you will either have to prove that you can bring more value or build something of your own. If you feel limited by the market in France, then a few US companies will be interested in hiring you as a contractor. You will then have to pay a bunch of taxes you didn't know about in France but your cash inflow will increase if you price yourself right.
In terms of technology, what everyone looks for is a general understanding of software (architecture, algorithms, data structure, etc) and then mastery in one or more languages. I'd recommend looking into a functional language (Scala, Elixir etc) to broaden your thinking as well.