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

src/build/docker/debian/Dockerfile

@@ -40,6 +40,7 @@ COPY ./dist/linux /src/dist/linux
 COPY ./images /src/images
 COPY ./include /src/include
 COPY ./pkg /src/pkg
+COPY ./po /src/po
 COPY ./nix /src/nix
 COPY ./vendor /src/vendor
 COPY ./build.zig /src/build.zig