Fix for translating language select boxes (#1393)

Create the choices when we instatiate the form rather than when we
define the class.  This makes them localized to the user's language.
Before once we localized them once, we always used the same strings.

Updated MultiVideoCreateSubtitlesForm to use the
subtitle_language_code and primary_audio_language_code fields it
inherits from its base class rather than defining the same fields
again.