From https://bugs.freebsd.org/244226 :
syslogd has code to convert all logged messages to "safe" strings.
At the moment, the code converts control characters to "^x" sequences and ALL 8-bit characters to "M-x" sequences. This means that printable characters in character sets other than ASCII are converted and so do not display as expected when viewing the logs.
This patch adds LC_CTYPE locale support to syslogd and changes the "safe" conversion code to examine the logged characters using mbrtoc32() and to use iswgraph() to test if a character needs converting to safe sequences.
It also uses vis() to do the conversion which is similar to OpenBSD but which means control chars become \C-x and non-graphical 8-bit chars become \M-x.