macOS
You can change language preferences on a per-app basis by going to System Preferences -> Language & Region -> Apps
Click the + icon to change language for a specific application.
If you're using a version of macOS that is older than 11.0 then you will not see this option and will have to change the system language, or use a third party application in order to change language on a per-app basis.
You can also change the language of Capture One in the Terminal. Launch the Terminal and type the following command:
defaults write com.captureone.captureone15 AppleLanguages '(en)'
NOTE: Substituting 15 to 14, 13, 12, 11, etc. will make it work for the corresponding version of Capture One. You can change en to another language code (see the table above) according to the ISO 639 standard.
For example, here is how you would change Capture One 21 to German from English:
defaults write com.captureone.captureone14 AppleLanguages '(de)'
Restart Capture One to see the changes.
For Windows
Activate Capture One application, then go to Edit -> Preferences -> General.
Once you've chosen the preferable language, restart the application: