Any advice on which programming language I should practice on? In college we mainly use C++, unless we get a project that demands otherwise- other than that I don't really practice coding that much, I was thinking of checking online contests on C++ and stuff and participating.
Honestly, mobile dev is really hot right now - since mobile platforms are very new, there aren't a lot of senior engineers with the skills. Android devs are especially in-demand, since it's such a pain to deal with the SDK and Android apps weren't profitable until recently (therefore, few people in the industry learned it).
The most important thing, though, is to be a good software engineer - someone who's very good at writing software
as part of a team. As a software engineer, readability and testability matters way, way more than writing technically perfect code. If your code is perfect but unreadable, the first person to touch it will break it, and the software will get worse over time. If your code is buggy but easy to read and unit-tested, it will get better over time.
Things like short methods, descriptive method names, methods with minimal side effects - this isn't taught well in university. But don't worry: once you get into the workforce, you'll pick up these habits naturally from looking at code written by senior engineers.

(being "multilingual" also helps... since different languages focus on different patterns, you can learn useful techniques by practicing in multiple languages)