Add support for languages

[martinheidegger]

It is good to know which presentation is held in which language.

I would like to append the current line to support in <language-code> at the end of a entry line. i.e.:

10:00-11:00 A by B in ja

{
  summary: 'A',
  person: 'B',
  lang: 'ja'
}

Which should only be supported in the first line of the entry:

10:00-11:00 A by B
    more title in ja
    has more text

should not extract the language.

{
  summary: 'A\nmoretitle in ja\nhas more text',
  person: 'B'
}

If there is a unknown language used: an error should be thrown.