The solutions provided will actually return the current region of the device – not the currently selected language. These are often one and the same. However, if I am in North America and I set my language to Japanese, my region will still be English (United States). In order to retrieve the currently selected language, you can do:

NSString * language = [[NSLocale preferredLanguages] firstObject];

This will return a two letter code for the currently selected language. “en” for English, “es” for Spanish, “de” for German, etc. For more examples, please see this Wikipedia entry (in particular, the 639-1 column):

List of ISO 639-1 codes

Then it’s a simple matter of converting the two letter codes to the string you would like to display. So if it’s “en”, display “English”.

EDIT

Worth to quote the header information from NSLocale.h:

+ (NSArray *)preferredLanguages NS_AVAILABLE(10_5, 2_0); // note that this list does not indicate what language the app is actually running in; the [NSBundle mainBundle] object determines that at launch and knows that information

People interested in app language take a look at @mindvision’s answer