l10n-% is used for repacks, chrome-% for multi-locale packages.
toolkit/mozapps/installer/l10n-repack.py
i suspect your --with-l10n-base line is what's messing that up. i've never had to specify that... mach should grab what it needs and keep it in your ~/.mozbuild directory
$ ./mach build chrome-zh-CN
0:24.69 c:\mozilla-build\bin\mozmake.EXE -j8 -s chrome-zh-CN
0:24.86 mozmake.EXE[1]: warning: -j1 forced in submake: resetting jobserver mode.
0:24.96 mozmake.EXE[1]: *** No rule to make target 'chrome-zh-CN'. Stop.
0:24.96 mozmake.EXE: *** [F:/comm/mail/build.mk;39: chrome-zh-CN] Error 2
0:24.99 256 compiler warnings present.
Hey! Builds initiated with `mach build $A_SPECIFIC_TARGET` may not always work, even if the code being built is correct. Consider doing a bare `mach build` instead.
You could try copying and adapting https://searchfox.org/mozilla-ce ... s/Makefile.in#71-85 across to the mail equivalent file, though you might also have to look at some of the blame and history, and see if the patches that added those lines needed to change other files as well.
You might also need to compare the two files and make sure that the other parts are similar as well.