The Java class java.util.Locale has a set of public static final fields for languages/countries for direct references to specific locales. Other locales can easily be used via the appropriate constructors.
However, the selection of values seems rather arbitrary to me.
The languages are (in the order in which they appear in the source): English, French, German, Italian, Japanese, Korean, Chinese, Simplified Chinese, Traditional Chinese
The countries are: France, Germany, Italy, Japan, Korea, China, PRC, Taiwan, UK, US, Canda and Canada French (for the locale fr_CA).
Does anyone have an idea (or even a guess) how those languages/countries where chosen?
Those codes are defined by ISO standards 3166 for country codes and 639 for language codes. See also the W3C Internationalization Activity page.
Edit:
Re-reading the question, the OP already knew this :-)
According to the Sun article Internationalization: Understanding Locale in the Java Platform those values are chosen as examples and it even tells us:
Because so few premade locales exist, you should probably just avoid these static objects altogether
In Java 19+, use the convenient Locale.of methods.
Locale.of( "en" , "CA" ) for English in Canada.Locale.of( "fr" , "CA" ) for French in Canada.Locale.of( "en" , "US" ) for English in United States.