Spring boot で多言語化する(i18n)
Spring bootでは
https://ti-tomo-knowledge.hatenablog.com/entry/2018/06/16/225947
public class Application {
    /**
     * i18n用
     */
    @Bean
    public MessageSource messageSource() {
        ResourceBundleMessageSource messageSource = new ResourceBundleMessageSource();
        messageSource.setBasename("messages");
        messageSource.setDefaultEncoding("UTF-8");
        return messageSource;
    }