When using Maven, properties files should be located inside src/main/resources not in src/main/java (see here)
So, for instance, if you have the following definition in your faces-config.xml (for using the msgs variable in your facelet pages) :
<resource-bundle>
<base-name>i18n.PanneauPrincipal</base-name>
<var>msgs</var>
</resource-bundle>
Or if you load the resource files programmatically :
ResourceBundle bundle = ResourceBundle.getBundle("i18n.PanneauPrincipal", locale);
Then the PanneauPrincipal_en.properties file should be located in the following directory :
src/main/resources/i18n