"hundreds of thousands of utterances" from volunteers for each specific language. Some languages like Swedish don't follow a normal set of pronunciation rules which requires Google to bring in native speakers to pronounce a long list of words.
The new languages include Basque, Bulgarian, Catalan, European Portuguese, Finnish, Galician, Hungarian, Icelandic, Norwegian, Romanian. Serbian, Slovak and Swedish. The update has just started rolling out so if you haven't yet received, don't panic just yet. Google says that the everyone will receive the update by next week. The Mountain View maestros also wanted to tell everyone that Google Voice Search works with only one language at a time and said that you might need to fiddle with your language settings to get one of the new languages to work.
source: Google via AndroidPolice