What would be the best approach to translate an app?

@Gordon I like your idea if replacing eg LANG("Hello") on upload. You could search for the translation first in an app-specific file and if not found in a common file.
However I still don't like each app to require to have the user choose a language on upload (also this make updating the app to a new version problematic, currently if you update locale, you get always the default language not the language you selected on the upload). As I said, best would be to obtain the language set in the locale file on the device.