Introduction
With a good understanding of both Ruby and JavaScript, you've gone through the process of learning a new language twice already. We recommend you continue to improve the understanding of these two languages - a clear depth of knowledge on one or two languages is often more advantageous than knowing a little about many languages. You're also already past the major hurdles of learning Ruby and JavaScript and can grasp more advanced concepts in these langauges.
However, depending on your career goals and the opportunities available to you, having an understanding of a third popular programming language can help.
One of the most popular languages currently is Python and its popularity continues to rise. Like Ruby, you can use Python to build a backend for a full-stack application. Similar to Ruby on Rails, Python has a couple of web frameworks for making it easier to spin up projects: Django and Flask
Python is also widely used in other fields such as data science. Having some familiarity in the language can help you stand out as a candidate.
Where to Start Learning Python
There are many free resources online for learning Python. We recommend first checking out the documentation available at python.org to get setup. After you've got everything setup take a look around for tutorials that fit your needs. JetBrains has recently made their Python IDE free to use and the JetBrains Academy provides tutorials for all skill levels using their IDE.
The Hitchhiker's Guide to Python is another good comprehensive resource and there are many other free option that can be found with a quick Google search.