$form->setTranslator($this->translator->domain('forms.testForm'));
$form->addError('onSuccess.errors.error1'); // should work
$form->addError('forms.testForm.onSuccess.errors.error1'); // should not work
$form->addError(new Phrase('onSuccess.errors.error1')); // should work
$form->addError(new Phrase('forms.testForm.onSuccess.errors.error1')); // should not work
via http://forum.nette.org/cs/23047-kdyby-translator-preklad-form-adderror-a-this-flashmessage#p155723