That makes no sense. It's very easy to build a REST API in Rails in which case you can connect to that API from services written in other languages. Conversely, you can easily call out to other services' APIs from Rails.

In fact, I'll go so far as to say building a few choice services in particularly performant (lower-level) languages alongside a main Rails monolith is the best organizational pattern for larger applications and enterprise deployments. You get all the benefits of Ruby & Rails for most happy paths, as well as the benefits of "This goes to 11!!!" performance for the critical paths which need that.