I have tested over here and it worked in my application. Something you could check is if you have the LocalizationKey defined in both French and English. And of course, that you have "Generated XML" after you created your Text Values.
Cheers.
My application supports 3 languages. I set the PropertyToLocalize with Title.
When I run my application, the title doesn't update and stay in french language whereas the language is english one.
I made tests with the normal GroupBox and it works.
Is there a bug ?
David