Symfony2 - translatable field - The class 'Gedmo\Translatable\Entity\Translation' was not found in the chain configured namespaces

You need to configure the translatable Entity to use as well. In config.yml:

orm:
    (....)
    mappings:
        translatable:
            type: annotation
            is_bundle: false
            prefix: Gedmo\Translatable\Entity
            dir: "%kernel.root_dir%/../vendor/gedmo/doctrine-extensions/lib/Gedmo/Translatable/Entity"
            alias: GedmoTranslatable

Update for Symfony 5 : Configure the /config/packages/doctrine.yaml file and add the following

orm:
    (....)
    mappings:
        translatable:
            type: annotation
            is_bundle: false
            prefix: Gedmo\Translatable\Entity
            dir: '%kernel.project_dir%/vendor/gedmo/doctrine-extensions/src/Translatable/Entity'
            alias: GedmoTranslatable