TRANSLATION

$ ls
extension.js  locale  metadata.json  po  prefs.js  schemas  stylesheet.css
$ xgettext --from-code=UTF-8 --output=po/example.pot *.js

$ mkdir ~/.local/share/gnome-shell/extensions/example@wiki.gnome.org/po/
$ mkdir ~/.local/share/gnome-shell/extensions/example@wiki.gnome.org/locale/

$ msginit --no-translator \
          --input=po/example.pot \
          --output-file=po/fr.po --locale=fr
$ ls po/
example.pot  fr.po

$ mkdir -p locale/fr/LC_MESSAGES/
$ msgfmt po/fr.po -o locale/fr/LC_MESSAGES/example.mo
$ ls locale/fr/LC_MESSAGES/
example.mo

// If you're using GJS in a standalone application, you should do this yourself
String.prototype.format = imports.format.format;

// Two examples that both output: A string with a digit "5" and and string "test"
let num = 5;
let str = 'test';

// With String.prototype.format()
'A string with a digit "%d" and string "%s"'.format(num, str);

// With native template literals, doesn't work with xgettext
'A string with a digit "${num}" and string "${str}"';

******

See Scripts 

bin/pot_create.sh - scan files and update pot file
bin/po_update.sh  - update po files per language and generate binary locale .mo file