Probably affects 6.12 also, though haven't checked.
As noted by Martin Grigorov in
The code for creating a ValidationError is at
As you can see the cause's message is not used :-/
The problem is:
ValidationError error = new ValidationError();
it should be:
ValidationError error = new ValidationError(cause.getMessage());
Later at https://github.com/apache/wicket/blob/master/wicket-core/src/main/java/org/apache/wicket/validation/ValidationError.java?source=c#L241 Wicket will look for i18n keys and fallback to the message.