As this problem appears to be a Gnulib bug that was fixed by Bruno Haible here:
https://lists.gnu.org/r/bug-gnulib/2020-09/msg00097.html I'm closing the grep bug report.
- bug#41999: Grep 3.4, Solaris 11.3 and FAIL: test-nl_langinf... Jeffrey Walton
- bug#41999: Grep 3.4, Solaris 11.3 and FAIL: test-nl_la... Paul Eggert
