Appointments+ multi-language setup, only 1 language accepted.

I have a multisite with 2 subsites, each is targeted at a different language. I have added the corresponding wordpress language files, so one subsite is in dutch and the other in french.

I used poedit to create translations for Appointments+ (for both languages). I uploaded the NL_nl and FR_fr files to the language directory.
The dutch language works perfectly, but in the french subsite english is chosen.
For some reason the french language file is not accepted.

Any idea on how i can fix this?