Setting Arabic numbering system locale doesn't show Arabic numbers

YES, I did it! After reading Locale's JavaDoc carefully, I was able to produce the required locale:

Locale arabicLocale = new Locale.Builder().setLanguageTag("ar-SA-u-nu-arab").build();

which is equivalent to:

Locale arabicLocale = new Locale.Builder().setLanguage("ar").setRegion("SA")
                   .setExtension(Locale.UNICODE_LOCALE_EXTENSION, "nu-arab").build();

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Note that I am using something called (Unicode locale/language extension):

UTS#35, "Unicode Locale Data Markup Language" defines optional attributes and keywords to override or refine the default behavior associated with a locale. A keyword is represented by a pair of key and type.

The keywords are mapped to a BCP 47 extension value using the extension key 'u' (UNICODE_LOCALE_EXTENSION).

The extension key for numbers is (nu) and the value I used is (arab).

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You can see a list of all extension keys here.