Symfony translation inside validation won't work - validation

Question

If I try to use translations inside validation like described here
http://inchoo.net/tools-frameworks/symfony2-translating-validator-messages/
I get always
not.blank.username
as output on Error. It seems like symfony didn't find the message-translation. I placed all my translationfiles inside app/ressources/translations and they are named as
messages.de.yml
....
What am I doing wrong? Do I have to place the translationfiles insde each Bundle?
Kind regards
EDIT: Problem ist clear and fixed, but when I try to use "MinLength" I get an strange error:
Attempted to load class "MinLength" from namespace
"Symfony\Component\Validator\Constraints" in /var/www/symfony/webprojekt/vendor/symfony/symfony/src/Symfony/Component/Validator/Mapping/Loader/AbstractLoader.php line 64.
Do you need to "use" it from another namespace?
I already included
use Symfony\Component\Validator\Mapping\ClassMetadata;
use Symfony\Component\Validator\Constraints\NotBlank;
use Symfony\Component\Validator\Constraints\Email;
use Symfony\Component\Validator\Constraints\MinLength;
use Symfony\Component\Validator\Constraints\MaxLength;
AM I missing something?

Answer 1

To translate validation messages you need to create files with the following name structure inside translations folder:
validators.LANG.yml
Also, you can check in your config.yml, inside framework, if exists the translator:
framework:
translator: { fallback: %locale% }
This is required.

Answer 2

The accepted answer applies to using the magical default translation domain for the validator config. You can also override this in config/packages/validator.yaml
framework:
validation:
enable_annotations: true
translation_domain: 'validations'
Then in translations/ make a file validations.LANG.yaml, for example translations/validations.en.yaml. You don't need to do this but sometimes it's better to explicitly set these magical bit and know where they're coming from.