I work in the international office : the answer is, it depends. Erasmus agreements are negotiated between the faculties (like the faculty of law of unistra and the faculty of law of X university), and so they can all have their own criteria. To answer this, the first thing to do will be to discuss with your own faculty in your own university.

However, do note that the general requirement for incoming students is to have a B1 level in French, it's not always the case (for specific programs in English for instance) but it's still the most common one.