Honor MODELTRANSLATION_LANGUAGES in admin

[olivierdalang]

Hi !

It is currently not possible to have a language disabled for users, but still displayed in the admin. This however can be useful if you add a new translation to a site, and want editors to translate it before actually enabling it for visitors.

Example settings

LANGUAGES = [
    ("fr", _("French")),
    # ("en", _("English")),  # english translation not ready yet
]
MODELTRANSLATION_LANGUAGES = ["fr", "en"]

From a django-modeltranslation perspective, it looks like it works as expected (/enURL do not exist, but the *_en model fields are properly created). In wagtail-admin however, the *_en fields are displayed as regular field and can't be toggled like normal translations.

Thanks !