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”.
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