Assuming that you have created localizations for English and German, your app will use the language selected in Settings (this is the preferred option).
If you want to set the language directly from your application:
[[NSUserDefaults standardUserDefaults] setObject:[NSArray arrayWithObjects:@"en", nil] forKey:@"AppleLanguages"];
will set the language for your application only to English (for German, I assume the key is "de"). You should also:
[[NSUserDefaults standardUserDefaults] synchronize];
This will not take effect until your application restarts.
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