In the section for LC_NUMERIC, locale(5) says: decimal_point followed by the string that will be used as the decimal delim- iter when formatting numeric quantities. .. without constraint on what the string may include. It appears this is actually constrained by processors to be a single character, as at https://code.woboq.org/userspace/glibc/locale/programs/ld-numeric.c.html#229 It is not clear if that is part of the spec: if so, I think this should be made explicit in the man page; if not, it is probably still worth noting that some applications (such as localedef) require it. I have not checked, but it seems likely that thousands_sep has the same constraint, and would need similar treatment. Initially reported for Ubuntu at https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/1898902, they recommended I re-post upstream.
The manual pages are maintained by the Linux man-pages project. I submitted a patch: https://lore.kernel.org/linux-man/87zh4vdc7d.fsf@oldenburg2.str.redhat.com/T/