Thx, alright I wanted to standardize my project on 639-2 and so for example inititate Thread.CurrentThread.CurrentUICulture = new CultureInfo("eng-US") but this leads to an ArgumentException because "eng-US" is not valid. On http://msdn.microsoft.com/en-us/library/system.globalization.cultureinfo.aspx the Culture identifier are listed, so long story short i have to use 639 and 639-2 only for the cases where the language code is not available with 2 letters (e.g. kok-IN).
But this is not such a big deal, because i can use ThreeLetterISOLanguageName property when i wanted to make the connection to a database where i am only working wih 639.2.
Anyways in my eyes it would have been nicer only to work with 639-2