Route names are for internal usage and naming not for translations. Routes must not have names in every language rather they must each have a unique name attached to a specific route. Your users won't ever see a string of contact.index as long as you pass it in route('contact.index') or any other helper which then makes it into a HTML link containing the URL not the route name.
tl;dr;: Routes should have a fixed name and never be translated into each language your system supports as they are not translations or attached to app locale.