Missing localisation initialisation code

[aral]

Initialisation code similar to the following is missing from the Application class constructor:

GLib.Intl.setlocale (LocaleCategory.ALL, "");
GLib.Intl.bindtextdomain (Constants.GETTEXT_PACKAGE, Constants.LOCALEDIR);
GLib.Intl.bind_textdomain_codeset (Constants.GETTEXT_PACKAGE, "UTF-8");
GLib.Intl.textdomain (Constants.GETTEXT_PACKAGE);

Without it, translations that use gettext / _() appear to work (in meson builds, not in Flatpak) but ngettext doesn’t (in either).

More details: small-tech/watson#16