This is two part: Use the Creative Cloud Desktop App to install the language you want besides what you already have. If you have English, install French.
The second part is to go to Photoshop Preferences > Interface > UI Language and you should see English and French in the dropdown menu. Choose the one you want and restart Photoshop.
Creative Cloud Preferences:
Then Install French:
When finished, open Photoshop, and go to Preferences > Interface > UI Language
and choose the language. Mine is English or Japanese. Restart.
