If proving competency in French language is a requirement, you can take a test to obtain one of the following certificates:

Elementary Diploma of the French Language
Diplôme d'Études en Langue Française (DELF)
The Diplôme d'Études en Langue Française (DELF) certifies an elementary level of the French language (foreign students). This diploma also certifies the competence in French language for private and public institutions and for business activities.

Advanced Diploma of the French Language
Diplôme Approfondi de Langue Française (DALF)
The Diplôme Approfondi de Langue Française (DALF) certifies an advanced level of the French language (foreign students). This diploma also certifies the competence in French language for private and public institutions, for business activities, and it is recognised by French universities.

Test de Connaissance du Français (TCF)
The TCF is a standardized test of general French language, designed and graded by the CIEP (Centre international d'études pédagogiques). This exam is intended for non-native French speakers who, for professional, personal or academic reasons, wish to have their competency in French assessed and validated. At the outcome of the test, the CIEP issues an attestation (valid 2 for years) certifying the candidate's current level in French.

The following institutions offer test preparation courses to get the certificates mentioned above