ResourceBundleResourceI18nConcern is mutable should not have singleton.
There are no convenience ResourceBundleResourceI18nConcern constructors, and instead there is a static singleton ResourceBundleResourceI18nConcern.DEFAULT instance.
The problem is that ResourceBundleResourceI18nConcern is mutable; most importantly its locale can change.
There should not therefore be a static singleton default instance of ResourceBundleResourceI18nConcernProvider.
ResourceBundleResourceI18nConcernProvider should create a new instance of ResourceBundleResourceI18nConcernProvider, and not a default singleton instance.
There should be simpler, convenience ResourceBundleResourceI18nConcernProvider constructors so that they may be created for custom Csar configurations.