This issue can be reproduced by creating a Seam EAR project. The Web project will contain xhtml errors.
The problem is because I18nValidator is declared as a V2 validator, but doesn't implement the org.eclipse.wst.xml.core.internal.validation.eclipse.Validator.validate method.
org.eclipse.wst.xml.core.internal.validation.eclipse.Validator is executed with the default indicatNoGrammar value 1 (a warning) which throws errors.

A solution is to implement the Validator.validate method or set indicateNoGrammar to 0 (none).

BTW A validator by the name JSF2 Components Validator already exists.