Вы пытались переписать метод ModelAdmin.formfield_for_foreignkey(db_field, request, **kwargs)
, как описано в документации ?
class MyModelAdmin(admin.ModelAdmin):
def formfield_for_foreignkey(self, db_field, request, **kwargs):
if db_field.name == "translation":
kwargs["queryset"] = Translation.objects.filter(lang="en")
return super().formfield_for_foreignkey(db_field, request, **kwargs)