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By default, the user locale is detected from the incoming `Accept-Language` header. You can provide users the capability to switch locales by simply passing a parameter called `lang` to Grails as a request parameter:

[source,groovy]
----
/book/list?lang=es
----

Grails will automatically switch the user's locale and subsequent requests will use the switched locale.

To offer a language selector, the i18n plugin discovers the locales your application is actually translated into by scanning the classpath for `messages_&#42;.properties` bundles (plus the default locale, configurable via `grails.i18n.default.locale`). This list is exposed as the `AvailableLocaleResolver` bean and published to the servlet context under the `availableLocales` attribute. The `<g:localeSelect available="true"/>` tag renders a `<select>` limited to those locales, and you can iterate the list directly in a view to build your own switcher:

[source,xml]
----
<g:each in="${application.getAttribute('availableLocales')}" var="locale">
    <a href="?lang=${locale.toLanguageTag()}">${locale.getDisplayName(locale)}</a>
</g:each>
----

By default every `&#42;.properties` bundle on the classpath is considered, so locales contributed by plugins &mdash; whose bundles are namespaced (for example `spring-security-core_fr.properties`) &mdash; are listed alongside your application's own. Candidate suffixes are validated against the ISO language codes (and the ISO country codes when a country is present), so non-i18n properties files (such as `application.properties`) are ignored. To restrict discovery to your application's own `messages_&#42;.properties` bundles, set `grails.i18n.availableLocales.includePlugins=false`:

[source,yaml]
.grails-app/conf/application.yml
----
grails:
    i18n:
        availableLocales:
            includePlugins: false   # list only the application's own translations
----

Applications created with `grails create-app` include a ready-made language dropdown in the default layout that uses exactly this mechanism.

By default, Grails uses {springapi}org/springframework/web/servlet/i18n/SessionLocaleResolver.html[SessionLocaleResolver] as the `localeResolver` bean.

You can select a different resolver strategy with the `grails.i18n.localeResolver` configuration property, without declaring a bean:

[source,yaml]
.grails-app/conf/application.yml
----
grails:
    i18n:
        localeResolver: acceptHeader # session (default), cookie, acceptHeader or fixed
----

`session` and `cookie` are mutable, so the `?lang=` switch works with them: the incoming `Accept-Language` header sets the initial locale, and a `?lang=` request parameter then overrides it and is remembered for subsequent requests (in the session or a cookie). `acceptHeader` (the locale follows the incoming `Accept-Language` header on every request) and `fixed` are read-only, so the `?lang=` switch has no effect and is silently ignored — the header, or for `fixed` the `grails.i18n.default.locale` value (falling back to the JVM default locale), always wins.

You can change the default locale easily:

[source,groovy]
.grails-app/conf/spring/resources.groovy
----
import org.springframework.web.servlet.i18n.SessionLocaleResolver

beans = {
    localeResolver(SessionLocaleResolver) {
        defaultLocale= new Locale('es')
    }
}
----

Other `localeResolver` are available. For example, you could use save the switched locale in a Cookie:

[source,groovy]
.grails-app/conf/spring/resources.groovy
----
import org.springframework.web.servlet.i18n.CookieLocaleResolver

beans = {
    localeResolver(CookieLocaleResolver) {
        defaultLocale= new Locale('es')
    }
}
----

Or fix the locale:

[source,groovy]
.grails-app/conf/spring/resources.groovy
----
import org.springframework.web.servlet.i18n.FixedLocaleResolver

beans = {
    localeResolver(FixedLocaleResolver, new Locale('de'))
}
----
