Improves the filter_horizontal widget for many to many admin relationship to model using django-mptt and django-parler.

mptt_m2m_admin.py

# models.py
from mptt.models import MPTTModel, TreeForeignKey
from parler.models import TranslatableModel, TranslatedFields

from .managers import CategoryManager


class Category(MPTTModel, TranslatableModel):
    parent = TreeForeignKey('self', null=True, blank=True, related_name='children', db_index=True)

    translations = TranslatedFields(
        name = models.CharField(_('Name'), max_length=100),
        slug = models.SlugField(_('slug'), max_length=255, blank=True, allow_unicode=True),
        meta = {'unique_together': (('language_code', 'slug'),)}
    )

    objects = CategoryManager()
    ...

    
class Article(TranslatableModel):
    categories = models.ManyToManyField('Category', blank=True)
    ...


# mptt_m2m_admin.py
from django.forms import ModelMultipleChoiceField
from django.utils.encoding import smart_text


class MPTTMultipleChoiceField(ModelMultipleChoiceField):
    def label_from_instance(self, obj):
        level = getattr(obj, getattr(self.queryset.model._meta, 'level_attr', 'level'), 0)
        return u'%s %s' % ('-'*level, smart_text(obj))


# forms.py
from django.contrib.admin.widgets import FilteredSelectMultiple
from parler.forms import TranslatableModelForm

from .mptt_m2m_admin import MPTTMultipleChoiceField
from .models import Article, Category


class ArticleForm(TranslatableModelForm):
    categories = MPTTMultipleChoiceField(
        Category.objects.all(), 
        widget=FilteredSelectMultiple('Categories', False),
        required=False
    )

    class Meta:
        model =  Article
        fields = '__all__'


# admin.py
from django.contrib import admin
from parler.admin import TranslatableAdmin

from .forms import ArticleForm
from .models import Article


@admin.register(Article)
class ArticleAdmin(TranslatableAdmin):
    form = ArticleForm