This is a follow-up from ICU-20575:
I wonder if it would make sense to add a test case for this to the CI builds?
Yes, this would make a great test case. Could probably create one subtest and run it, something such as:
env LANG=C make check CINTLTEST_OPTS=/cloctst/test-c-locale
where there's a test that checks LANG and expects some result.