Aperture will use the language that is set in the System Preferences, in the "Language & Region" tab.
- Quit Aperture and other open apps..
- Launch System Preferences and open the Language & Region" tab.
- Drag the language you want to the top of the list of preferred languages. Aperture only supports English, French, German, Japanese, Chinese. Pick one of these languages.
Then close the panel and restart your Mac. Relaunch Aperture.