When a method of a JAX-RS resource class is invoked due to e.g. a GET or a POST request, then the method's parameters are validated via bean validation.
However, the language of violation messages is always the default Locale, and not calculated using the header parameter accept-language, e.g. accept-language=de.
http://beanvalidation.org/1.1/spec contains an example for a class LocaleSpecificMessageInterpolator so that validation happens with a dedicated locale.