Here is a patch that adds in NLS.
There are some opportunities to improve this. For example, we could
move the list of languages from the meson.build files into separate
LINGUAS files, which could be shared with the makefile-based build
system. I need to research this a bit more.
Also, this only covers the build and install phases of the NLS process.
The xgettext and msgmerge aspects I haven't touched at all. There is
more to research there as well.
The annoying thing is that the i18n module doesn't appear to have a way
to communicate with feature options or dependencies, so there isn't a
way to tell it to only do its things when some option is enabled, or
conversely to check whether the module found the things it needs and to
enable or disable an option based on that. So right now for example if
you explicitly disable the 'nls' option, the binaries are built without
NLS but the .mo files are still built and installed.
In any case, this works for the main use cases and gets us a step
forward, so it's worth considering.