Being fully comfortable writing in Ruby and JavaScript is very beneficial - a clear depth of knowledge on one or two languages is often more advantageous than knowing a little about many languages. You're also past the major hurdles of learning Ruby and JavaScript and can grasp more advanced concepts in these langauges. If you don't feel comfortable in either, continue to practice by building projects in them.
If you've reached a point where you feel ready to learn take on a completely new challenge, it may be time to consider learning a third programming language. One of the most popular languages today is Java. Java is widely used for application development and for larger, enterprise-scale systems, but can also be used as a backend for a web application.
The Java programming language is a bit of a departure from what you've seen so far with Ruby and JavaScript when it comes to syntax. To give you an idea, Ruby and JavaScript have some noticeable syntatic differences when it comes to creating and using Arrays, but the two languages are much more similar than how Arrays are handled in Java.
W3 Schools offers tutorials on the basics of Java that are a good for getting familiar with the language's syntax. For more advanced practice, JetBrains has recently made their Java IDE free to use and provides a Java tutorial to help you build your first application. There are also many free tutorials available online to help you.
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