Configure django-cms for multiple languages

[Baschdl]

See http://docs.django-cms.org/en/latest/how_to/languages.html#multilingual-support-how-to

This includes

• [x] Use {% language_chooser "myapp/language_chooser.html" %} (http://docs.django-cms.org/en/latest/how_to/languages.html#display-a-language-chooser-in-the-page)
• [x] Integrate handling of the user’s language preference with our current way
• [x] Move translations from .po file to translated version in django-cms
• [ ] Also check translations, we saw multiple wrong ones when reviewing webpack-js

[Baschdl]

Our language chooser works as intended

[bjrne]

The language chooser might actualy not work as expected. The built-in language switcher of django-cms does not work with our page so far. Switching the language with our switcher works visually, but when editing the page with django-cms, it seems that weird glitches arise. Needs further investigation.