Implementing Internationalization (i18n)

@Bean
MessageSource messageSource() {
  ReloadableResourceBundleMessageSource ms = new ReloadableResourceBundleMessageSource();
  ms.setBasename("classpath:messages");
  ms.setDefaultEncoding("UTF-8");
  ms.setCacheSeconds(60);
  ms.setFallbackToSystemLocale(false);
  return ms;
}

greeting=Hello, {0}!
items.count={0,choice,0#no items|1#one item|1<{0,number,integer} items}

@Bean
LocaleResolver localeResolver() {
  AcceptHeaderLocaleResolver r = new AcceptHeaderLocaleResolver();
  r.setSupportedLocales(List.of(Locale.ENGLISH, Locale.FRENCH, Locale.forLanguageTag("es")));
  r.setDefaultLocale(Locale.ENGLISH);
  return r;
}

@Bean
LocaleResolver localeResolver() {
  CookieLocaleResolver r = new CookieLocaleResolver();
  r.setCookieName("APP_LOCALE");
  r.setCookieMaxAge(Duration.ofDays(365));
  r.setDefaultLocale(Locale.ENGLISH);
  return r;
}

@RestController
public class GreetController {
  private final MessageSource ms;
  public GreetController(MessageSource ms) { this.ms = ms; }
  @GetMapping("/hello/{name}")
  public String hello(@PathVariable String name, Locale locale) {
    return ms.getMessage("greeting", new Object[]{name}, locale);
  }
}

<p th:text="#{greeting('World')}">Hello</p>
<label th:text="#{form.email}">Email</label>

@Override
public void addInterceptors(InterceptorRegistry r) {
  LocaleChangeInterceptor lci = new LocaleChangeInterceptor();
  lci.setParamName("lang"); // ?lang=fr
  r.addInterceptor(lci);
}

public class PathLocaleResolver implements LocaleResolver {
  private final Pattern P = Pattern.compile("^/(\\w{2})/.*$");
  @Override public Locale resolveLocale(HttpServletRequest req) {
    Matcher m = P.matcher(req.getRequestURI());
    return m.matches() ? Locale.forLanguageTag(m.group(1)) : Locale.ENGLISH;
  }
  @Override public void setLocale(HttpServletRequest r, HttpServletResponse s, Locale l) {}
}