Symfony2 contains a lot of validators out of the box, but it is not very clearly documented how to use custom messages when validation fails for these standard validators. After some investigation, I found the validation messages in Symfony/vendor/symfony/src/Symfony/Bundle/FrameworkBundle/Resources/translations/. It is very easy to override a message. Just look up the message you wish to modify (e.g. ‘This value should not be blank’), create your own validators.[language].yml file (or .xliff if you prefer) in your bundle, and add this message as a key with your custom message as value :
# validators.en.yml This value should not be blank: This is a required field
That’s all 
Thanks !