Have a war application which deploys and runs correctly on EAP 6.4. When deploying on WIldfly 10.1.0, none of the message bundle lookups work, resulting in ?zxcv? text on JSF pages.
This application has the following in faces-config.xml:
<application>
<locale-config>
<default-locale>en</default-locale>
<supported-locale>en_US</supported-locale>
<supported-locale>en</supported-locale>
</locale-config>
<message-bundle>com.rfdinc.efile.bp.resources.CombinedMessageResources</message-bundle>
<resource-bundle>
<base-name>com.rfdinc.efile.bp.resources.CombinedMessageResources</base-name>
<var>msg</var>
</resource-bundle>
</application>

Here, com.rfdinc.efile.bp.resources.CombinedMessageResources is a Java class which extends java.util.ResourceBundle and implements most of the methods of java.util.ResourceBundle.

This technique works correctly under EAP 6.4. Under Wildfly 10.1.0, however, none of the class methods ever get called. This was verified by placing logger messages throughout com.rfdinc.efile.bp.resources.CombinedMessageResources.

Replacing <message-bundle> and <base-name> with a 'properties' file works, but this particular Java class allows us to combine multiple property files into a single resource bundle.