gettext Icelandic tests fail on macOS with locale FR

Hi,

I was thinking of contributing a patch, and noticed the following test breakages on master, macOS with locale FR:

*** t0204-gettext-reencode-sanity.sh ***
# lib-gettext: Found 'is_IS.UTF-8' as an is_IS UTF-8 locale
# lib-gettext: Found 'is_IS.ISO8859-1' as an is_IS ISO-8859-1 locale
ok 1 - gettext: Emitting UTF-8 from our UTF-8 *.mo files / Icelandic
ok 2 - gettext: Emitting UTF-8 from our UTF-8 *.mo files / Runes
not ok 3 - gettext: Emitting ISO-8859-1 from our UTF-8 *.mo files / Icelandic
#
#     printf "TILRAUN: Halló Heimur!" | iconv -f UTF-8 -t ISO8859-1 >expect &&
#     LANGUAGE=is LC_ALL="$is_IS_iso_locale" gettext "TEST: Hello World!" >actual &&
#     test_cmp expect actual
#
ok 4 - gettext: impossible ISO-8859-1 output
ok 5 - gettext: Fetching a UTF-8 msgid -> UTF-8
not ok 6 - gettext: Fetching a UTF-8 msgid -> ISO-8859-1
#
#     LANGUAGE=is LC_ALL="$is_IS_iso_locale" gettext "TEST: ‘single’ and “double” quotes" >actual &&
#     grep "einfaldar" actual &&
#     grep "$(echo tvöfaldar | iconv -f UTF-8 -t ISO8859-1)" actual
#
ok 7 - gettext.c: git init UTF-8 -> UTF-8
ok 8 - gettext.c: git init UTF-8 -> ISO-8859-1
# failed 2 among 8 test(s)

Warm regards,
Ram