Windows 10 modern-ui apps wrong language
I've solved my problem by putting in the languages settings the French language at the top. After some restarts all my modern ui apps are in French.
Registered just to post this, hope it helps others who got fustrated with it as well.
Under Control Panel > Language > Advanced Settings > Override for Windows display language
Defaults follows your language setup, you can choose to select what language you wan't to show here.