I would think this would affect any program that calls strftime(3) with abbreviated weekday name (%a),
for example ps(1):

https://svnweb.freebsd.org/base/head/bin/ps/print.c?revision=335023&view=markup#l411

410             } else if (now - k->ki_p->ki_start.tv_sec < 7 * 86400) {
411                             (void)strftime(buf, buflen, "%a%H  ", tp);

then it would just print out a 五 here instead of 周五:

% env LANG=zh_CN.UTF-8 ps auwwx | grep '[s]bin/sshd'
root 26612  0.0  0.0 20784  100  -  IsJ  五12    0:00.02 /usr/sbin/sshd

% env LANG=en_US.UTF-8 ps auwwx | grep '[s]bin/sshd'
root 26612  0.0  0.0 20784  100  -  IsJ  Fri12   0:00.02 /usr/sbin/sshd

In D18689#398817, @loader wrote:

I would think this would affect any program that calls strftime(3) with abbreviated weekday name (%a),
for example ps(1):

https://svnweb.freebsd.org/base/head/bin/ps/print.c?revision=335023&view=markup#l411

410             } else if (now - k->ki_p->ki_start.tv_sec < 7 * 86400) {
411                             (void)strftime(buf, buflen, "%a%H  ", tp);

then it would just print out a 五 here instead of 周五:

% env LANG=zh_CN.UTF-8 ps auwwx | grep '[s]bin/sshd'
root 26612  0.0  0.0 20784  100  -  IsJ  五12    0:00.02 /usr/sbin/sshd

% env LANG=en_US.UTF-8 ps auwwx | grep '[s]bin/sshd'
root 26612  0.0  0.0 20784  100  -  IsJ  Fri12   0:00.02 /usr/sbin/sshd

The current representation is more problematic when the caller supplies a buffer that is shorter (for example, cal(1)), where the truncation would keep only one Chinese character. For example:

$ LANG=zh_CN.UTF-8 cal
      十二月 2018        
周 周 周 周 周 周 周  
                   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 29  
30 31

After patch:

      12月 2018
日 一 二 三 四 五 六
                   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 29
30 31

On macOS Mojave, the weekday behavior of ps(1) is similar to FreeBSD (after patch).