在resources创建文件

messages_zh_CN.properties（中文），写入

#Unicode转码
hello=\u4f60\u597D

messages_en_US.properties（英文），写入

hello=hello world

结构如图

创建配置类

@Configuration
@EnableAutoConfiguration
@ComponentScan
public class LocaleConfig implements WebMvcConfigurer {

    //Cookie
    @Bean
    public LocaleResolver localeResolver() {
        CookieLocaleResolver localeResolver = new CookieLocaleResolver();
        localeResolver.setCookieName("localeCookie");
        //设置默认区域
        localeResolver.setDefaultLocale(Locale.ENGLISH);
        //设置cookie有效期.
        localeResolver.setCookieMaxAge(3600);
        return localeResolver;
    }

    @Bean
    public LocaleChangeInterceptor localeChangeInterceptor() {
        LocaleChangeInterceptor lci = new LocaleChangeInterceptor();
        // 参数名
        lci.setParamName("lang");
        return lci;
    }

    @Override
    public void addInterceptors(InterceptorRegistry registry) {
        registry.addInterceptor(localeChangeInterceptor());
    }

}

测试

@RestController
public class TestController {

    @Autowired
    private MessageSource messageSource;

    @GetMapping(path = "/test2",produces = "text/html;charset=utf-8")
    public Object test2(){
        Locale locale = LocaleContextHolder.getLocale();
        String hello = messageSource.getMessage("hello", null, locale);
        return hello;
    }
    
}

访问
http://127.0.0.1:8080/test2?lang=zh_CN
http://127.0.0.1:8080/test2?lang=en_US