Have the list of available languages on a page

[fiammybe]

It would be very interesting if we would be able to determine which languages are on the page, if only to make the language selector widget more relevant. This would mean having to go through the content of the page, and making a list of the different language codes encountered.

If we would have #273 already, we could imagine pre-calculating this at the moment of storing the page texts, and storing it as a property. I don't think other ways are available to store metadata for an object without changing the module that handles the object.