fix empty attribute options on unsupported language

steps to reproduce

-   with a fresh user (who has not yet set user.language) set the
    browser language to french (or any other language except for en and de)
-   go to the attributes tab in subject management
-   examine the options for handedness

expected behavior

-   "Inconnu", "Right", "Left", "Ambidextrous"

actual behavior

-   "Inconnu"

solution

See https://django-parler.readthedocs.io/en/stable/quickstart.html#filtering-translations