You can look at job descriptions posted by companies like Google, FB etc. All of these big tech companies which hire new grads have requirement of one object oriented language. So from the perspective of getting job, you just need to know one language very well. Java in your case should be perfectly fine. Apart from languages, the most important thing which would be tested are coding skills, algorithm & data structure knowledge and finally your CS fundamentals like knowledge of OS etc. For becoming proficient in coding & algorithms you can participate in competitive programming like codechef, topcoder, etc. You can also try to apply for Google summer of code to get experience doing software development projects.
If you still wanted to know which language would be helpful apart from Java, I would suggest Python since it is used currently at lot of new tech companies which are coming up and so learning it can be beneficial.