Commit 0a36787e4c1f ("locales: separate unicode from other locales", https://cgit.freebsd.org/src/commit/?id=0a36787e4c1fa0cf77dcf83be0867178476e372b, https://github.com/freebsd/freebsd-src/commit/0a36787e4c1fa0cf77dcf83be0867178476e372b) split the locale category Makefiles into unicode and non-unicode variants (e.g. share/msgdef/Makefile and share/msgdef_unicode/Makefile). During this split, several cross-encoding SAME entries were dropped because the source was a UTF-8 locale and the target was an ISO8859 locale, so they didn't fit cleanly into either file.

As a result, on a clean 15.0 install, the following setlocale() calls return NULL:
setlocale(LC_MESSAGES, "nl_BE.ISO8859-1")   → NULL
setlocale(LC_MESSAGES, "nl_BE.ISO8859-15")  → NULL
setlocale(LC_MESSAGES, "hr_HR.ISO8859-2")   → NULL
setlocale(LC_MESSAGES, "sr_RS.ISO8859-2")   → NULL
setlocale(LC_NUMERIC,  "zh_TW.Big5")        → NULL

The locale directories exist (created by other categories like LC_COLLATE, LC_CTYPE, LC_MONETARY, LC_TIME), but the specific category files are missing.

The pre-split Makefile had cross-encoding entries like:
SAME+=  nl_NL.UTF-8 nl_BE.ISO8859-1
SAME+=  sr_RS.UTF-8@latin hr_HR.ISO8859-2
But these were dropped during the split.

For zh_TW.Big5, its LOCALES+= entry was dropped from numericdef/Makefile even though zh_TW.Big5.src still exists in the tree.

Proposed fix: Add non-Unicode-only entries that stay within each Makefile's domain:
# share/msgdef/Makefile
SAME+=  nl_NL.ISO8859-15 nl_BE.ISO8859-15
SAME+=  nl_NL.ISO8859-15 nl_BE.ISO8859-1
SAME+=  sl_SI.ISO8859-2 hr_HR.ISO8859-2
SAME+=  sl_SI.ISO8859-2 sr_RS.ISO8859-2

# share/numericdef/Makefile
LOCALES+=  zh_TW.Big5

The ISO8859 source files (nl_NL.ISO8859-15/LC_MESSAGES, sl_SI.ISO8859-2/LC_MESSAGES) are byte-identical to their UTF-8 counterparts because LC_MESSAGES content (yes/no patterns) is pure ASCII.

Note: Systems upgraded from FreeBSD 13 (or earlier) may not see the failure because stale symlinks from the pre-split installation survive installworld, which I will file another bug for and link to this issue.