There has been a long tail of tests that implicitly depend on the locale of the user from which the test is run. The tests fail when run by a user with locale that is not "en" or simiilar.

Having a mechanism to set and propagate a locale within a test run would be helpful.

On most Unix systems the Posix environment variables LANG and LC_* are used to set system properties user.language and user.country that control the local. And Jtreg, on Unix, when it spawns processes propagates LC_ALL.
On Windows and on macOS, the Posix variables are not used to establish the locale of the Java runtime.

A mechanism to define user.language and user.country properties would make it easier to establish the test environment.