gtk: add localization support, take 3 (#6004)

This is my third (!) attempt at implementing localization support. By
leveraging GTK builder to do most of the `gettext` calls, I can avoid
the whole mess about missing symbols on non-glibc platforms.

Added some documentation too for contributors and translators, just for
good measure.

Supersedes #5214, resolves the GTK half of #2357

nix/devShell.nix

@@ -35,6 +35,7 @@
   gobject-introspection,
   libadwaita,
   blueprint-compiler,
+  gettext,
   adwaita-icon-theme,
   hicolor-icon-theme,
   harfbuzz,
@@ -129,6 +130,9 @@ in
         # wasm
         wabt
         wasmtime
+
+        # Localization
+        gettext
       ]
       ++ lib.optionals stdenv.hostPlatform.isLinux [
         # My nix shell environment installs the non-interactive version