Added Serbian Cyrillic, Serbian Latin and Croatian, thanks Zsolt!

This pull request was originally against the master branch, now moved to 2.5.x.

It was necessary to create the file install_minimal/upgrades/airtime-2.5.2/data/upgrade.sql and modify airtime_mvc/build/sql/defaultdata.sql slightly. The format of locale names in defaultdata.sql has also been tidied up, and there are some corrections for the Hungarian localization.