en: more fixes

en: more fixes

- Language picker in index, mac and windows: as in workshops;
  NOTE: Language names are in their own language; they shouldn't be
  translated. The "current" class should be moved from English to the
  relevant language.

- Images in section 5: different alt attributes for different images;
  NOTE: Image descriptions are a little more specific in some translations.
  If there were several different translations for the same original string,
  we wouldn't be able to make correct PO files, should we ever need them.