When specifying the Locale directly, everything works as expected, when not, the default locale is used, which is incorrect. Ex.

log.info("SeamResourceBundle.getBundle w/o passing Local: " + SeamResourceBundle.getBundle("ECISConfig").getString("ecis.hello"));
log.info("SeamResourceBundle.getBundle w passing Local: " + SeamResourceBundle.getBundle("ECISConfig", LocaleSelector.instance().getLocale()).getString("ecis.hello"));

Produces:
15:08:45,438 INFO [TestAction] SeamResourceBundle.getBundle w/o passing Local: Hello from ECISConfig_en.properties
15:08:45,439 INFO [TestAction] SeamResourceBundle.getBundle w passing Local: Hello from ECISConfig_en_US_tmwa.properties

When both should be saying "Hello from ECISConfig_en_US_tmwa.properties" because the locale and resource bundle are defined in components.xml as:

<international:locale-selector locale-string="en_US_tmwa" />

<core:resource-loader>
<core:bundle-names>
<value>messages</value>
<value>ECISConfig</value>
</core:bundle-names>
</core:resource-loader>