Simply include a parameter in the request, either from a link or a form, that sets the requst_locale parameter to the language and (optionally) counry code to use.
<ul>
<li><html:link action="/Locale?language=en">English</html:link></li>
<li><html:link action="/Locale?language=ja" useLocalEncoding="true">
Japanese</html:link></li>
<li><html:link action="/Locale?language=ru" useLocalEncoding="true">
Russian</html:link></li>
</ul>
The default interceptor stack includes an i18n interceptor that watches for the request_locale parameter. The parameter name is configurable.