If you want to search in particular file only then you can use <s:i18n>
tag.
Using getText
method if your action extends ActionSupport
<s:i18n name="labels">
<s:property value="getText('some.label')" />
</s:i18n>
or using <s:text>
tag
<s:i18n name="labels">
<s:text name="some.label" />
</s:i18n>
But you can also include more than one properties files to custom default resource bundles:
<constant name="struts.custom.i18n.resources" value="images, labels" />