To add a new language or update the existing ones:

If your language isn't here, add it to the LINGUAS file.

Use meson in the gtk directory to create the build files, ie:
  $ meson build

Change to the build directory and run 'ninja snes9x-gtk-update-po'
  $ cd build
  $ ninja snes9x-gtk-update-po

This will update all the .po files in this directory with the latest strings 
and create any new files for those in LINGUAS.

Use any method desired to translate the strings in the .po file.