This works fine
export LANG=en_GB.utf8
for i in {01..12}
do
mnd=$(date -d "2020-$i-01" +%B)
date -d "01-${mnd:0:3}-2020" +%B
done
But this doesn't work for months may, october, and desember
export LANG=nn_NO.utf8
for i in {01..12}
do
mnd=$(date -d "2020-$i-01" +%B)
date -d "01-${mnd:0:3}-2020" +%B
done
It is also worth noting the difference when your not taking a
substring of the month, works fine with english and more fails with
norwegian.
