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>
<s:url id="en" action="Welcome">
<s:param name="request_locale">en</s:param>
</s:url>
<s:a href="%{en}">English</s:a>
</li>
<li>
<a href="<s:url action="Welcome">
<s:param name="request_locale">ja</s:param>
</s:url>">Japanese</a>
</li>
<li>
<a href="<s:url value="Welcome.do">
<s:param name="request_locale" value="'ru'" />
</s:url>">Russian</a>
</li>
</ul>
The default interceptor stack includes an i18n interceptor that watches for the request_locale parameter. The parameter name is configurable.