When an application registers it's own TextProvider via 'struts.xworkTextProvider' in struts.xml, it is not used for validation errors of Model-Members of ModelDriven actions.
Łukasz commented on the mailing list:
> There is a bug in DelegatingValidatorContext#makeTextProvider method -
> it should first try to use parent before creating new instance of
> TextProvider (base on TextProviderFactory but without injecting
> dependencies :)