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.
<h3>Language Options</h3>
<ul>
<li><a href="<s:url action="Welcome?request_locale=en"/>">English</a></li>
<li><a href="<s:url action="Welcome?request_locale=ja"/>">Japanese</a></li>
<li><a href="<s:url action="Welcome?request_locale=ru"/>">Russian</a></li>
</ul>
The default interceptor stack includes an i18n interceptor that watches for the request_locale parameter. The parameter name is configurable.