思路

主要就是利用runtime hook系统的方法,宏定义为NSLocalizedString(key, comment)，方法为[NSBundle.mainBundle localizedStringForKey:(key) value:@"" table:nil]，设置语言的时候传入正确的语言代码并找到相应的bundle，进而得到想要的字符串。当然已经显示的页面还要自己处理页面的刷新，比如更新rootViewController。

#import <Foundation/Foundation.h>

@interface NSBundle (Language)

+ (void)setLangage:(NSString *)language;//zh-Hans,en,zh-Hant...

@end

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

static NSString *languageKey = @"chosenLanguage";

@implementation NSBundle (Language)

+ (void)load {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        Class class = [self class];
        SEL originalSelector = @selector(localizedStringForKey:value:table:);
        SEL swizzledSelector = @selector(yx_localizedStringForKey:value:table:);
        
        Method originalMethod = class_getInstanceMethod(class, originalSelector);
        Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
        
        BOOL didAddMethod = class_addMethod(class, originalSelector, method_getImplementation(swizzledMethod), method_getTypeEncoding(swizzledMethod));
        if (didAddMethod) {
            class_replaceMethod(class, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod));
        } else {
            method_exchangeImplementations(originalMethod, swizzledMethod);
        }
    });
}

- (NSString *)yx_localizedStringForKey:(NSString *)key value:(nullable NSString *)value table:(nullable NSString *)tableName {
    NSString *chosenLanguage = [[NSUserDefaults standardUserDefaults] objectForKey:languageKey];
    chosenLanguage = chosenLanguage ? chosenLanguage: @"Base";
    NSBundle *chosenBundle = [NSBundle bundleWithPath:[[NSBundle mainBundle] pathForResource:chosenLanguage ofType:@"lproj"]];
    chosenBundle = chosenBundle ? chosenBundle: NSBundle.mainBundle;
    NSString *result = [chosenBundle yx_localizedStringForKey:key value:value table:tableName];
    return result;
}

+ (void)setLangage:(NSString *)language {
    [[NSUserDefaults standardUserDefaults] setObject:language forKey:languageKey];
    [[NSUserDefaults standardUserDefaults] synchronize];
}

@end