There is not an easy way to do this with the Route Resolver, yet. The language switcher in that project generates links using the
altternate_languages property on each document, but that property doesn't include enough information to build complex URLs. So, the only option would be to perform an API query for each alternate language and get the nested URL.
However, we currently have multi-language support for the Route Resolver in development. I can't give you a timeline for this right now, but I'm syncing with the dev team on it at some point this week, so I can give you an update next week.
In the meantime, I would suggest performing additional calls to get the URLs for the alternate languages, after the initial API query. Let me know if I can help you to implement that as a temporary workaround.