Java SE (Java Platform, Standard Edition)

Hi everyone,
Most of the java applications fail to start on Turkish locale machines and Turkish developers are accustomed to start applications with -Duser.language jvm option. My question is about the value that -Duser.language takes. If i use just tr, applications fail to work. Instead i have to give tr_TR or tr-TR but according to internationalization documentations, language parameter should be just tr. For a java application i tried the following combinations:

-Duser.language=tr (fails to start)
-Duser.language=tr -Duser.country=TR (fails to start)
-Duser.language=en -Duser.country=US
-Duser.language=tr_TR
-Duser.languate=tr-TR

What is the reason that the first two combinations doesn't work and the last three works? Also what is the correct syntax for language parameter?

Thanks