Senior Software Engineer at DataXu
It really depends on what you are interested in. If it's testing there are a lot of relevant languages you can learn. Python and Java come to mind. If you are interested in android development you will need to learn Java. For iPhone you will need to learn objective C. Java is always a safe choice because it has a lot of options if you decide that you no longer wanted to do mobile development, whereas objective C opportunities are going to be almost exclusively iPhone.
If it were me I would start off with Java. It is a widely used language that has a lot of opportunity in mobile development, testing, and other areas. Also the OOP (object oriented programming) skills you will gain along the way are transferable to any other language you might learn in the future.