Use method swizzling to change both language and locale at runtime, to use in your unit tests.

NSBundle+TTTOverrideLanguage.h

#import <Foundation/Foundation.h>

@interface NSBundle (TTTOverrideLanguage)

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

+ (void)ttt_resetLanguage;

@end

NSBundle+TTTOverrideLanguage.m

#import "NSBundle+TTTOverrideLanguage.h"
#import "NSObject+TTTSwizzling.h"

@implementation NSBundle (TTTOverrideLanguage)

+ (void)load
{
    [self ttt_swizzleLanguageBundles];
}

+ (void)ttt_swizzleLanguageBundles
{
    [self ttt_swizzleInstanceMethod:@selector(localizedStringForKey:value:table:)
                    withReplacement:@selector(ttt_localizedStringForKey:value:table:)];
}

static NSBundle *ttt_languageBundle = nil;

+ (void)ttt_overrideLanguage:(NSString *)language
{
    NSString *path = [[NSBundle mainBundle] pathForResource:language ofType:@"lproj"];
    ttt_languageBundle = [NSBundle bundleWithPath:path];
}

+ (void)ttt_resetLanguage
{
    ttt_languageBundle = nil;
}

- (NSString *)ttt_localizedStringForKey:(NSString *)key value:(NSString *)value table:(NSString *)tableName NS_FORMAT_ARGUMENT(1);
{
    if (ttt_languageBundle)
    {
        return [ttt_languageBundle ttt_localizedStringForKey:key value:value table:tableName];
    }

    return [self ttt_localizedStringForKey:key value:value table:tableName];
}

@end

NSLocale+TTTOverrideLocale.h

#import <Foundation/Foundation.h>

@interface NSLocale (TTTOverrideLocale)

+ (void)ttt_overrideRuntimeLocale:(NSLocale *)locale;

+ (void)ttt_resetRuntimeLocale;

@end

NSLocale+TTTOverrideLocale.m

#import "NSLocale+TTTOverrideLocale.h"
#import "NSObject+TTTSwizzling.h"
#import <objc/runtime.h>

@implementation NSLocale (TTTOverrideLocale)

+ (void)load
{
    [self ttt_swizzleLocales];
}

static NSLocale *ttt_locale = nil;

+ (void)ttt_overrideRuntimeLocale:(NSLocale *)locale
{
    ttt_locale = locale;
}

+ (void)ttt_resetRuntimeLocale
{
    ttt_locale = nil;
}

+ (void)ttt_swizzleLocales
{
    [self ttt_swizzleClassMethod:@selector(autoupdatingCurrentLocale) withReplacement:@selector(ttt_autoupdatingCurrentLocale)];
    [self ttt_swizzleClassMethod:@selector(currentLocale) withReplacement:@selector(ttt_currentLocale)];
    [self ttt_swizzleClassMethod:@selector(systemLocale) withReplacement:@selector(ttt_systemLocale)];
}

+ (id /* NSLocale * */)ttt_autoupdatingCurrentLocale
{
    return ttt_locale ?: [self ttt_autoupdatingCurrentLocale];
}

+ (id /* NSLocale * */)ttt_currentLocale
{
    return ttt_locale ?: [self ttt_currentLocale];
}

+ (id /* NSLocale * */)ttt_systemLocale
{
    return ttt_locale ?: [self ttt_systemLocale];
}

@end

NSObject+TTTSwizzling.h

#import <Foundation/Foundation.h>

@interface NSObject (TTTSwizzling)

+ (void)ttt_swizzleClassMethod:(SEL)original withReplacement:(SEL)swizzled;

+ (void)ttt_swizzleInstanceMethod:(SEL)original withReplacement:(SEL)swizzled;

@end

NSObject+TTTSwizzling.m

#import <objc/runtime.h>
#import "NSObject+TTTSwizzling.h"

@implementation NSObject (TTTSwizzling)

+ (void)ttt_swizzleClassMethod:(SEL)original withReplacement:(SEL)swizzled
{
    method_exchangeImplementations(class_getClassMethod(self, original), class_getClassMethod(self, swizzled));
}

+ (void)ttt_swizzleInstanceMethod:(SEL)original withReplacement:(SEL)swizzled
{
    method_exchangeImplementations(class_getInstanceMethod(self, original), class_getInstanceMethod(self, swizzled));
}

@end

Hi!

I'm not expert in swizzling. Why do we have recursive call if for example ttt_locale is nil? Would it be stackoverflow?

Nope. The code is correct. Review swizzling again.

With modernized Objective-C id /* NSLocale * */ can be replaced with instancetype.