Hi, It seems the entry.desktop from the source messages folder is not installed, that way the language does not show up in the systemsettings. running autogen.sh nl manually in the source dir and after that make install in the build dir solves the problem.
I can confirm this, I'm just not sure why it happens (other than autogen.sh not working in the face of symlinks). The build directory is basically symlinks to the corresponding files in the source directory so that the source directory doesn't have stray files laying around. But the resultant CMakeLists.txt files are mostly broken in this situation from what I can tell. The real fix would be to l10n/scripts/autogen.sh IMO but if I ever get time I'll see if I can work around it (although a working patch would be accepted as well, if it maintains srcdir != builddir :)
*** Bug 190815 has been marked as a duplicate of this bug. ***
Recent update to autogen.sh should have fixed this issue.