As per the issue raised at appium repo and question asked here JAVA is the most used language.
Advantages and disadvantage:
1. Static vs Dynamic language
2. Java has very active client development,( which I think because the majority of people using JAVA)
3. Java has TestNG. (My personal opinion, other languages have powerful X unit framework too but I personally like TestNG)
4. Java very good Integration with IDE's because of being a Statically typed language.
There might be many other things.
However, it all depends on which language you are good at if you are better in Ruby compare to JAVA you should definitely go with Ruby.
I have previously worked with JAVA, currently working with python and loving it.