Cal ignores LC_ALL=en_GB.utf8 and starts week on Sunday

[Toni Kunic]

Ubuntu 16.04.3 LTS, and Ubuntu 17.10.

16.04$ apt-cache policy bsdmainutils
bsdmainutils:
  Installed: 9.0.6ubuntu3
  Candidate: 9.0.6ubuntu3
  Version table:
 *** 9.0.6ubuntu3 500
        500 http://ca.archive.ubuntu.com/ubuntu xenial/main amd64 Packages
        100 /var/lib/dpkg/status

17.10$ apt-cache policy bsdmainutils
bsdmainutils:
  Installed: 9.0.12+nmu1ubuntu1
  Candidate: 9.0.12+nmu1ubuntu1
  Version table:
 *** 9.0.12+nmu1ubuntu1 500
        500 http://ca.archive.ubuntu.com/ubuntu artful/main amd64 Packages
        100 /var/lib/dpkg/status

====================

Expected: With the en_GB.utf8 locale, cal should start on Monday.

====================

Observed behaviour: cal fails to observe locale, always starting weeks on Sunday.

$ LC_ALL=en_GB.utf8 cal
   February 2018
Su Mo Tu We Th Fr Sa
             1 2 3
 4 5 6 7 8 9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24
25 26 27 28

$ LC_ALL=en_US.utf8 cal
   February 2018
Su Mo Tu We Th Fr Sa
             1 2 3
 4 5 6 7 8 9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24
25 26 27 28

====================

For comparison, ncal behaves correctly:

$ LC_ALL=en_GB.utf8 ncal
    February 2018
Mo 5 12 19 26
Tu 6 13 20 27
We 7 14 21 28
Th 1 8 15 22
Fr 2 9 16 23
Sa 3 10 17 24
Su 4 11 18 25

$ LC_ALL=en_US.utf8 ncal
    February 2018
Su 4 11 18 25
Mo 5 12 19 26
Tu 6 13 20 27
We 7 14 21 28
Th 1 8 15 22
Fr 2 9 16 23
Sa 3 10 17 24