You can localize the error message defined for individual regular expressions.
For HTML UI based applications, the bundle keys for error messages should be present in java bundle files for server side validations and in javascript bundles for client side validations.
For Web UI Framework based applications, the bundle keys for error messages should be present in java bundle files for server side validations and in javascript bundles for client side validations.
For RCP based applications, the bundle keys for error messages should be present in java bundle files for both server and client side validations.