landscape-client-ui-install: Dialog not i18ned

These bits need to use gettext for proper internationalization:

    link = "<a href=\"https://landscape.canonical.com\">Find out more...</a>"
    dialog.set_markup(
        "<big><b>%s</b></big>\n\n%s\n\n%s\n\n%s" % (
            "Landscape client",
            "Landscape is an easy to use systems management and monitoring "
            "service offered by Canonical that helps administrators manage "
            "multiple machines efficiently.", link,
            "You need to install Landscape client to be able to configure it. "
            "Do you want to install it now?"))
    dialog.set_title("Install Landscape client?")
    button = dialog.add_button("Install", Gtk.ResponseType.YES)

plus the package needs to install the corresponding mo files. ( They will be stripped out from the actual binary package, but will get into the language packs).