While knowing various programming languages is certainly valuable and is a factor in hiring, it’s not the first thing I look for when I interview candidates. Something that I like to see is enthusiasm and dedication for the role. I don’t want someone who is using the TA role as a way into the company, because what they really want to do is character art, or something else. I look for someone who doesn’t mind providing support to others, because it happens a lot. I don’t want a hermit where all they want to do is write tools, and not be bothered by a TA’s other responsibilities. I also look for someone who is open to others ideas. Since you are a student, one way you can display these sorts of traits is to provide support to others in your class, and provide information on what you did. An example could be that you had a class project to create a small animated short. You provided technical support by writing a custom tool that did <fill in the blank here>. Something along those lines. Not only will that experience be good for your resume, but it will also give you some perspective into what a TA does as well as talking points, for whomever you interview with.
In terms of learning a new language, whenever I want to pick up a new programming language, the easiest way for me to learn is to choose a small project and write it in that language. For instance, a project could be relevant to current or past school work that you have.
Hope this helps.