My app is localized.
But now some users want to open the app in a different language than the system language.
I found a way by executing this command in a Shell:
defaults write com.my_company.my_app AppleLanguages '("en")'
This works fine and my_app opens in english (independently from the system language)
But the bad thing is, as long as my_app is open, every application will open in english.
If I close my_app everything is back to normal.
EDIT: I was mistaken! It works as it should! 
When opening the app, use this shell script to read the status:
defaults read com.my_company.my_app AppleLanguages
and set to default, by using this:
defaults remove com.my_company.my_app AppleLanguages
If anybody is interested...
