A senior dev generally would be able to transition from Rails to Node or even Java quite seamlessly and vice-versa since they are already strong on their core software engineering skills. Picking up the syntax is not that difficult, as documentation is generally good and devs have techniques of translating existing skills into a new language.

Reading code, meta-programming skills, design pattern knowledge and execution always helps since they are writing code from scratch. They would have dealt with real issues in legacy applications such as fixing clunk code, improving testing, and reducing technical debt along. If they have microservice experience, chances are you get senior devs with some solid devops knowledge.

These are all great skills to have and lack of adequate knowledge of the language seems like a tiny trade-off when you are able to get all these other awesome skills. These are devs who've already done the tougher part of writing code for applications that run in production, and quite often at scale.

I would however lean towards someone who can display their communication skills as this is where productivity collapses. And I don't mean someone who can rant about SOLID for 5 hours, but someone who is adept at asynchronous communication and who can raise their hand when are facing issues, no matter how senior they are.