If the 'altBundle' attribute isn't specified but 'altKey' is, then the default resource bundle is used instead.
Resource bundles contain key/value pairs for specific locales and are used for internalization. Go here for more details. Note that Jahia finds the .properties file associated to a given bundleKey by doing a lookup in resourcebundles_config.xml located in \WEB-IN\etc\config. The resourcebundles_config.xml file is generated automatically during template deployment. For example, given the the following resourcebundles_config.xml:
<?xml version="1.0" encoding="UTF-8" ?>
<registry>
<resource-bundle>
<key>myjahiasite_CORPORATE_PORTAL_TEMPLATES</key>
<file>jahiatemplates.Corporate_portal_templates</file>
</resource-bundle>
</registry>
Then given a bundleKey of myjahiasite_CORPORATE_PORTAL_TEMPLATES, the key/value lookup will be in Corporate_portal_templates.propreties (located in directory \WEB-INF\classes\jahiatemplates) for the default language; or for example in Corporate_portal_templates_de.propreties if the current user's locale is German.