Unfortunately, there’s no official way provided by Apple for this purpose. Let’s look at two methods for solve this problem.

Method 1

Apple provides a way to specify application specific language, by updating the “AppleLanguages” key in NSUserDefaults. For example:

[[NSUserDefaults standardUserDefaults] setObject:@"fr" forKey:@"AppleLanguages"];
[[NSUserDefaults standardUserDefaults] synchronize];

For working this method, you’ll have to set it before UIKit initialized.

#import <UIKit/UIKit.h>
#import "AppDelegate.h"

int main(int argc, char * argv[]) {
    @autoreleasepool {
        [[NSUserDefaults standardUserDefaults] setObject:@"fr" forKey:@"AppleLanguages"];
        [[NSUserDefaults standardUserDefaults] synchronize];
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
    }
}

The problem of this method is that the app has to be relaunched to take effect.

Method 2

The solution is to swap the mainBundle of our application as soon as user changes their language preferences inside the app.

See the category for NSBundle.

Header:

#import <Foundation/Foundation.h>

@interface NSBundle (Language)

+ (void)setLanguage:(NSString *)language;

@end

Implementation:

#import "NSBundle+Language.h"
#import <objc/runtime.h>

static const char kBundleKey = 0;

@interface BundleEx : NSBundle

@end

@implementation BundleEx

- (NSString *)localizedStringForKey:(NSString *)key value:(NSString *)value table:(NSString *)tableName
{
    NSBundle *bundle = objc_getAssociatedObject(self, &kBundleKey);
    if (bundle) {
        return [bundle localizedStringForKey:key value:value table:tableName];
    }
    else {
        return [super localizedStringForKey:key value:value table:tableName];
    }
}

@end

@implementation NSBundle (Language)

+ (void)setLanguage:(NSString *)language
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        object_setClass([NSBundle mainBundle],[BundleEx class]);
    });
    id value = language ? [NSBundle bundleWithPath:[[NSBundle mainBundle] pathForResource:language ofType:@"lproj"]] : nil;
    objc_setAssociatedObject([NSBundle mainBundle], &kBundleKey, value, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

@end

In this method a problem that may arise is updating elements on active screens. You can reload your rootViewController from our application delegate, will always work reliably.

- (void)reloadRootViewController
{
    AppDelegate *delegate = [UIApplication sharedApplication].delegate;
    NSString *storyboardName = @"Main";
    UIStoryboard *storybaord = [UIStoryboard storyboardWithName:storyboardName bundle:nil];
    delegate.window.rootViewController = [storybaord instantiateInitialViewController];
}

All code you can see on github. With simple example.

Please, use for free and like it.

Note: in example project by default the app uses method #2. You can disable this. Just comment define USE_ON_FLY_LOCALIZATION.