Integration of PlaceholderField with django-hvad (using translations with PlaceholderField)

This is related to #1123

Fixes two things:

1. plugin editor js gets language from django-hvad current language tab (title on a span). This way plugin gets added in the correct language, if using django-hvad. Related to KristianOellegaard/django-hvad#76  

2. As stated in #1123 ,  admin currently shows all placeholder languages. I have tried to implement the proposed solution from ojii (only admin part). But maybe is not the correct way and should be revised. 

It works well for me (django 1.4 and django-cms develop trunk)

*note*:  javascript language selection will need pull request KristianOellegaard/django-hvad#76 accept (or similar) in django-hvad.