here a short info on how to change the language from Capture One Pro 6. The program is using the operating systems language. Lets say your system is in German but you want the english version of C1 so you can do a quick workaround.
Open a terminal and do the following:
$ cd /Applications/Capture\ One.app/Contents/Resources/
$ mv German.lproj German._lproj_
Do the renaming in Finder. Right Click in the application folder on Capture One and select show contents. Then go to Contents/Resources and rename.
To go back to your systems language just rename back.