Lang 26 test case does not fail for all system locales

[AndrewC19]

The test for bug Lang-26 (testLang645) only fails for particular system locales. For example, if your system locale is en_US, the test will fail. However, if your system locale is en_GB, the test will pass.

The test can be modified to ensure that it always catches the bug, regardless of your own system locale by adding the line Locale.setDefault(new Locale("en", "US")); to the start of the test.

[rjust]

@markro49 @mernst Looks like this change would be in line with other changes we made to ensure reproducibility for Java 11.

If this change allows us to remove the requirement to set TZ for external use of D4J artifacts, I think we should go for it.