Allow man(1) to search translated manpages in pure $man_lang subdirectory

[fluffy]

Summary

Currently our man(1) utility search manages only in $lang_country.charset, $lang.charset and, as failover, in $en.charset which is historical behavior.
But nowadays most of ported software uses only $lang subdirectories for translated manpages instead of use full locale notation, for example, share/ja/man1, share/de/man1, share/ru/man1. In such cases, our man(1) utility cannot find any translated manpages and silently fallback to english version.

Add $man_lang in search path allows man(1) search and use translated manpages under historical and modern path traditions

[ziaee]

usr.bin/man/man.1

245

This would need to be added to the list directly above this that says "If set, man will search for

locale specific manual pages using the following logic:"