How to read 'List separator' from OS in Java?

Without resorting to a platform specific solution I think that the best approach to take is going to be to allow users to specify their preference of list separator within your own application. Either in a preferences panel, a dialog box on export or via an optional command line argument.

From comments of this answer:

Reading the OS-specific setting is a need I have to meet.

So what if OSs other than Windows don't have such a setting?

I suggest you read it from registry on Windows (as alluded here): Read/write to Windows Registry using Java. On other platforms just use a good default, and perhaps, at least on Unix, also support configuring it via a custom environment variable (which you document well): How can my java code read OS environment variables?.

My gut feeling that OSs universally do not have a (system-wide or user-specific) "List separator" setting may be wrong, of course, but I doubt that.

In addition to providing your own option to the user in your application you could try to guess what the list separator is.

I had a look at some locales in Windows and saw that the list separator is either ";" or ",". I've heard there is another character in some obscure locale, but have not seen it myself. So if you can make your code to handle both ";" and "," as list separators then you will probably cover majority of cases.

Also, it looks like when "," is used as a decimal separator, then "," is never used as a list separator. I guess this is otherwise numbers will be impossible to distinguish in a list: 1,2,3,4 could be 1.2, 3.4 or 1, 2.3 In these cases ";" is used as a list separator. Unfortunately the reverse is not true. Arabic has "." as a decimal symbol and ";" as a list separator.

So I think the rule that can be reasonably safely followed is:

if (decimalSeparator == ',') 
    then listSeparator = ';'
else if (decimalSeparator == '.') 
    then listSeparator = new char[] {';', ','}