That was helpful. So it's dynamic, therefore reasonably future proof. Nice job, Karl!

The answer I wanted on my system is:

› rpm -q --qf %{VERSION} glibc-locale
2.29

› for l in $(locale -a) ; do \
    LC_CTYPE=$l perl5.30.0 -Mlocale \
        -E'say $ENV{LC_CTYPE} if "i" ne lc "I"' ; \
    done
az_AZ
crh_UA
ku_TR
ku_TR.utf8
tr_CY
tr_CY.utf8
tr_TR
tr_TR.utf8
tt_RU@iqtelif
[download]

To save the interested reader a trip to the list of ISO 639 codes, that's Azeri, Crimean Tatar, Kurdish, Turkish, Tatar.