Don't rely on selection staying selected when doing crazy things to it
    
    LanguageSpoke inherits from the LanguageMixIn which is also used by the
    WelcomeLanguageSpoke and contains the treeview with available languages and the
    selection object. Obviously the selection doesn't stay selected when
    WelcomeLanguageSpoke is hidden.
    So in LanguageMixIn.initialize -- if there is nothing selected, try to use
    self.data.lang.lang value first and if it is None, fallback to prefered_language
    (English).
    
    Fixes the issue with no translations and default language selected on the
    SummaryHub before going to LanguageSpoke and back.