Getting current device language in iOS?

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".

Hope this helps someone that's looking to differentiate between region and currently selected language.

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

The selected answer returns the current device language, but not the actual language used in the app. If you don't provide a localization in your app for the user's preferred language, the first localization available, ordered by the user's preferred order, is used.

To discover the current language selected within your localizations use

[[NSBundle mainBundle] preferredLocalizations];

Example:

NSString *language = [[[NSBundle mainBundle] preferredLocalizations] objectAtIndex:0];

Swift:

let language = NSBundle.mainBundle().preferredLocalizations.first as NSString

Solution for iOS 9:

NSString *language = [[NSLocale preferredLanguages] objectAtIndex:0];

language = "en-US"

NSDictionary *languageDic = [NSLocale componentsFromLocaleIdentifier:language];

languageDic will have the needed components

NSString *countryCode = [languageDic objectForKey:@"kCFLocaleCountryCodeKey"];

countryCode = "US"

NSString *languageCode = [languageDic objectForKey:@"kCFLocaleLanguageCodeKey"];

languageCode = "en"