Ruby on Rails is sort of the basis for most any other modern MVC web framework. That is to say it inspired nearly every other web MVC framework, in most other languages. If you can use Rails you'll be able to use most anything else. Prior to Rails most web apps had proprietary frameworks powering them or no framework and were just an bunch of "pages" (endpoints) loosely strung together.
Rails also, for better or worse, has one of the easiest to use ORMs-- ActiveRecord. This helps new folks worry less about the intricacies of learning SQL (or whatever your persistence layer is) and more so about what writing a web service is really about.
Ruby as a language is also very expressive, is truly object oriented, and has all the right building blocks (that's kind of a pun actually), plus a very large amount of documentation and examples. It also is very good for meta programming which is what enables a lot of the rails magic (and fun).
At the end of the day learning more languages like PHP, Python, Java after Ruby is fairly trivial. I always tell noobs to learn whatever is useful for them, and stop worrying about what language.
I started with Ruby 10 years ago and now can write Java, PHP, Python, Scala, Elixir, Objective-C, and Swift; though I am by no means an expert in all of them. At some point you'll realize you've learned concepts and they're what is important. Not syntax.
P.s. don't let my username fool you ;) I'm just unfortunately hipster.
Edit: added some thoughts on why your first language isn't so relevant and a bit of clarity.
Edit 2: fixed absent minded use of rails where I should've written ruby.