Change Language application with English language by default, if you select an unsupported language (without restart application)

LanguageManager.h

#import <Foundation/Foundation.h>

typedef NS_ENUM(NSInteger, ELanguage)
{
    ELanguageEnglish,
    ELanguageGerman,
    ELanguageFrench,
    ELanguageRussian,
    ELanguageHebrew,
    
    ELanguageCount
};

@interface LanguageManager : NSObject


extern NSString *const LanguageCodes[];
extern NSString *const LanguageStrings[];
extern NSString *const LanguageSaveKey;


+ (void)setupCurrentLanguage;
+ (NSArray *)getLanguages;
+ (NSArray *)languageStrings;
+ (NSString *)currentLanguageString;
+ (NSString *)currentLanguageCode;
+ (NSInteger)currentLanguageIndex;
+ (void)saveLanguageByIndex:(NSInteger)index;
+ (BOOL)isCurrentLanguageRTL;

@end

LanguageManager.m

#import "LanguageManager.h"
#import "NSBundle+Language.h"

@implementation LanguageManager

NSString * const LanguageCodes[] = { @"en", @"de", @"fr", @"ru", @"he" };
NSString * const LanguageStrings[] = { @"English", @"German", @"French", @"Russian", @"Hebrew" };
NSString * const LanguageSaveKey = @"currentLanguageKey";

+ (void)setupCurrentLanguage
{
    NSString *currentLanguage = [[NSUserDefaults standardUserDefaults] objectForKey:LanguageSaveKey];
    if (!currentLanguage) {
        NSArray *languages = [[NSUserDefaults standardUserDefaults] objectForKey:@"AppleLanguages"];
        if (languages.count > 0) {
            currentLanguage = languages[0];
            [[NSUserDefaults standardUserDefaults] setObject:currentLanguage forKey:LanguageSaveKey];
            [[NSUserDefaults standardUserDefaults] synchronize];
        }
    }
    
    [[NSUserDefaults standardUserDefaults] setObject:@[currentLanguage] forKey:@"AppleLanguages"];
    [[NSUserDefaults standardUserDefaults] synchronize];
    
}

+ (NSArray *)getLanguages
{
    NSMutableArray *array = [NSMutableArray array];
    for (NSInteger i = 0; i < ELanguageCount; ++i) {
        [array addObject:LanguageCodes[i]];
    }
    return [array copy];
}

+ (NSArray *)languageStrings
{
    NSMutableArray *array = [NSMutableArray array];
    for (NSInteger i = 0; i < ELanguageCount; ++i) {
        [array addObject:NSLocalizedString(LanguageStrings[i], @"")];
    }
    return [array copy];
}

+ (NSString *)currentLanguageString
{
    NSString *string = @"";
    NSString *currentCode = [[NSUserDefaults standardUserDefaults] objectForKey:LanguageSaveKey];
    for (NSInteger i = 0; i < ELanguageCount; ++i) {
        if ([currentCode isEqualToString:LanguageCodes[i]]) {
            string = NSLocalizedString(LanguageStrings[i], @"");
            break;
        }
    }
    return string;
}

+ (NSString *)currentLanguageCode
{
    return [[NSUserDefaults standardUserDefaults] objectForKey:LanguageSaveKey];
}

+ (NSInteger)currentLanguageIndex
{
    NSInteger index = 0;
    NSString *currentCode = [[NSUserDefaults standardUserDefaults] objectForKey:LanguageSaveKey];
    for (NSInteger i = 0; i < ELanguageCount; ++i) {
        if ([currentCode isEqualToString:LanguageCodes[i]]) {
            index = i;
            break;
        }
    }
    return index;
}

+ (void)saveLanguageByIndex:(NSInteger)index
{
    if (index >= 0 && index < ELanguageCount) {
        NSString *code = LanguageCodes[index];
        [[NSUserDefaults standardUserDefaults] setObject:code forKey:LanguageSaveKey];
        [[NSUserDefaults standardUserDefaults] synchronize];
        [NSBundle setLanguage:code];
    }
}

+ (BOOL)isCurrentLanguageRTL
{
    NSInteger currentLanguageIndex = [self currentLanguageIndex];
    return ([NSLocale characterDirectionForLanguage:LanguageCodes[currentLanguageIndex]] == NSLocaleLanguageDirectionRightToLeft);
}

@end

main.m

#import <UIKit/UIKit.h>

#import "NSBundle+Language.h"
#import "LanguageManager.h"

int main(int argc, char * argv[])
{
    @autoreleasepool
    {
        NSString *currentLanguage = NSLocalizedString(@"currentLocale", @"");
        NSLog(@"currentLocale: %@",currentLanguage);
        NSString *language = [[NSLocale preferredLanguages] objectAtIndex:0];
        NSLog(@"Language: %@", language);
        NSArray *loc = [language componentsSeparatedByString:@"-"];
        if ([loc count]>0)
        {
            NSString *currentLang=@"";
            BOOL isLang = [[LanguageManager getLanguages] containsObject: loc[0]];
            if (isLang == YES)
            {
                currentLang=loc[0];
            }
            else
            {
                currentLang=@"en";
            }
            [[NSUserDefaults standardUserDefaults] setObject:currentLang forKey:LanguageSaveKey];
            [[NSUserDefaults standardUserDefaults] synchronize];
            [NSBundle setLanguage:currentLang];
            NSLog(@"currentLang: %@",currentLang);
        }
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
    }
}

NSBundle+Language.h

#import <Foundation/Foundation.h>

@interface NSBundle (Language)

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

@end

NSBundle+Language.m

#import "NSBundle+Language.h"
#import "LanguageManager.h"
#import <UIKit/UIKit.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]);
    });
    if ([LanguageManager isCurrentLanguageRTL]) {
        if ([[[UIView alloc] init] respondsToSelector:@selector(setSemanticContentAttribute:)]) {
            [[UIView appearance] setSemanticContentAttribute:
             UISemanticContentAttributeForceRightToLeft];
        }
    }else {
        if ([[[UIView alloc] init] respondsToSelector:@selector(setSemanticContentAttribute:)]) {
            [[UIView appearance] setSemanticContentAttribute:UISemanticContentAttributeForceLeftToRight];
        }
    }
    [[NSUserDefaults standardUserDefaults] setBool:[LanguageManager isCurrentLanguageRTL] forKey:@"AppleTextDirection"];
    [[NSUserDefaults standardUserDefaults] setBool:[LanguageManager isCurrentLanguageRTL] forKey:@"NSForceRightToLeftWritingDirection"];
    [[NSUserDefaults standardUserDefaults] synchronize];
    
    id value = language ? [NSBundle bundleWithPath:[[NSBundle mainBundle] pathForResource:language ofType:@"lproj"]] : nil;
    objc_setAssociatedObject([NSBundle mainBundle], &kBundleKey, value, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

@end