I've found the following solution:

NSDictionary *infoDict = [[NSBundle mainBundle] infoDictionary];
NSDictionary *extentionDictionary = infoDict[@"NSExtension"];
NSDictionary *extentionAttributesDictionary = extentionDictionary[@"NSExtensionAttributes"];
NSString *languageCode = extentionAttributesDictionary[@"PrimaryLanguage"];

I've found the following solution:

NSDictionary *infoDict = [[NSBundle mainBundle] infoDictionary];
NSDictionary *extentionDictionary = infoDict[@"NSExtension"];
NSDictionary *extentionAttributesDictionary = extentionDictionary[@"NSExtensionAttributes"];
NSString *languageCode = extentionAttributesDictionary[@"PrimaryLanguage"];