Allow the wx.ComboBox controls in an OptionsDialog to return another object rather than the displayed string

Example:
  choices = (('string A', 1), ('string B', 2), ('string C', 3))
  option_value = 3
This sets the displayed list entry to 'string C' and returns 1, 2 or 3.
Display native language names on the interface language selection control
Fix filling of empty translation files
Fix error message appearing for user sliders with a custom modulo on its label

The error popped up when refreshing the video preview. It seems that some
code was commited by mistake when scaling of user sliders was implemented