Language codes versus country codes

First reported by Bart.S: http://portableapps.com/node/18149#comment-113849

"Dear MuseScore developers, please change the czech locale code to 'cs' and use that for your next releases. Thanks! I believe 'cz' is used for license plates only :P"

Upon further investigation cz is the country code for Czech Republic (ISO 3166) and cs is the code for Czech language (ISO 631).

I propose we officially adopt IETF language tags since they are widely used on the Internet by HTML, XML, Wikipedia, etc. An example of IETF language tags is pt-BR (which we are already using). The first two letters are the language code (Portuguese) followed by the region code (Brazil) if necessary.