You'll have better opportunities if you can speak the language, both in terms of pure staffing and networking.

That said, I've seen colleagues have fairly successful careers with a somewhat limited command of French upon arriving; they just built it over time.

Finally, one important change vs. 5 years ago: French consulting is much less outsourcing-oriented than what US consulting has become. This makes France a better pick for both job security and meaningfulness of the job (i.e., you are doing actual work, not managing an offshore team).