In order to specify the default Unicode Locale extensions (JEP JDK-8177568), there should be following (internal) system properties for the java launcher command line option "-D"

- user.extensions
- user.extensions.display
- user.extensions.format

The value for these properties should take the form of BCP47's "extension" (https://tools.ietf.org/html/bcp47):

---
extension = singleton 1*("-" (2*8alphanum))

                                     ; Single alphanumerics
                                     ; "x" reserved for private use
 singleton = DIGIT ; 0 - 9
               / %x41-57 ; A - W
               / %x59-5A ; Y - Z
               / %x61-77 ; a - w
               / %x79-7A ; y - z
---

e.g. "u-ca-japanese" or "u-cu-jpy-nu-thai"