Force using english localized if current localized does not exists

Create a class:

CLocale.h

#undef NSLocalizedString#define NSLocalizedString(key, comment) \
[CLocale localizedString:(key)]
@interface CLocale : NSObject
+ (NSString*) localizedString:(NSString*)key;
@end

CLocale.m

#import "CLocale.h"
@implementation CLocale
+ (NSString*) localizedString:(NSString*)key
{
    NSArray* languages = [[NSUserDefaults standardUserDefaults] objectForKey:@"AppleLanguages"];
    NSString* currentLocale = [languages objectAtIndex:0];
    NSString* localized = [[NSBundle mainBundle] localizedStringForKey:key value:@"" table:nil];
    // if key and localized are same, also it is not english,
    // then force to use english language
    if ([localized compare:key] == NSOrderedSame && [currentLocale compare:@"en"] != NSOrderedSame)
    {
        NSBundle *bundle = [NSBundle bundleWithPath:[[NSBundle mainBundle] pathForResource:@"en" ofType:@"lproj"]];
        localized = NSLocalizedStringFromTableInBundle(key, nil, bundle, nil);
    }
    return localized;
}
@end

At last, include it into your pre-defined header (.pch)

#import "CLocale.h"

Done.