GLib doesn’t force any particular localization method upon its users. But since GLib itself is localized using the gettext() mechanism, it seems natural to offer the de-facto standard gettext() support macros in an easy-to-use form.

In order to use these macros in an application, you must include <glib/gi18n.h>. For use in a library, you must include <glib/gi18n-lib.h> after defining the GETTEXT_PACKAGE macro suitably for your library:

#define GETTEXT_PACKAGE "gtk4"
#include <glib/gi18n-lib.h>

For an application, note that you also have to call bindtextdomain(), bind_textdomain_codeset(), textdomain() and setlocale() early on in your main() to make gettext() work. For example:

#include <glib/gi18n.h>
#include <locale.h>

main (int argc, char **argv)
  setlocale (LC_ALL, "");
  bindtextdomain (GETTEXT_PACKAGE, DATADIR "/locale");
  bind_textdomain_codeset (GETTEXT_PACKAGE, "UTF-8");
  textdomain (GETTEXT_PACKAGE);

  // Rest of your application.

where DATADIR is as typically provided by Automake or Meson.

For a library, you only have to call bindtextdomain() and bind_textdomain_codeset() in your initialization function. If your library doesn’t have an initialization function, you can call the functions before the first translated message.

The gettext manual covers details of how to integrate gettext into a project’s build system and workflow.