"French" and the characters in the cipher text made me think of Vigenere. Need a key. Maybe 'confidentiel'? Yes.
[https://gchq.github.io/CyberChef/#recipe=Vigen%C3%A8re_Decode('confidentiel')&input=VnZyIGt0ZGsgdmwganZ0enN5SEJJe2ZuemNpZXZzfQ](https://gchq.github.io/CyberChef/#recipe=Vigen%C3%A8re_Decode('confidentiel')&input=VnZyIGt0ZGsgdmwganZ0enN5SEJJe2ZuemNpZXZzfQ)