How do I analyse the current demand for a language?

I've checked WhoIsHiring and LinkedIn, but the data isn't sharp enough. I'm rusty in all my languages and wouldn't pass interviews right now, except for data structures and algorithms in JS. I'm not an American and European companies rarely use that interview style.

I once did a booking.com assignment that took me too long to complete, eventhough I understood it. If I immersed myself in Java for a month, it wouldn't have been an issue.

I can choose any major popular language I want to focus on. Assuming my experience is about equal in all major popular programming languages, how would you go about analyzing the current demand?