Is sex selection illegal in France?

Yes. Sex selection is banned in France. Embryo tests are only for serious disease - never to pick a sex.

Party to the Council of Europe Oviedo Convention (ETS 164, in force 1 Dec 1999). Article 14 forbids using medically assisted procreation to choose a future child's sex, except to avoid a serious hereditary sex-related disease. A ratified treaty in force binds the state; France is on the CoE ratification chart; its Public Health Code limits embryo diagnosis to serious incurable genetic disease, so non-medical sex selection is unlawful.

Source: Oviedo; Code sante publique

Banned (50)Restricted (9)Allowed

France is one of 50 countries where sex selection is banned, and 9 more restrict it.
Its neighbours: banned in 5 of 8.
In Europe: banned in 34 of 39 countries.