#20 ISO-codes for language dirs

[Jan Fabry]

If there are many translations coming up, it might be
better to name the directories to their ISO-codes. This
is a more official way of doing it, and it also makes
it more understandible for non-English developers.

For example: English (standard) would be: en
English (as spoken in the US: en-US
Dutch: nl
Dutch (like it's spoken in Flanders): nl-BE