Well the are a few different languages used within the process of getting a game jam game complete within a time limit and rule restriction. Commonly I see a lot of C#, C++ and Java. If you have yet looked into game engines or game development tutorials or courses that you are interested in, I would strongly recommend doing so.
Here are a list of game engines. Some listed can be used with C https://en.wikipedia.org/wiki/List_of_game_engines
I recommend looking into what game jams are and which ever game development engine you are interested in. You can use your C knowledge and go for an engine that uses C, or quickly learn the basics of something else like C# From Unity's massive community very easily for example.
There are our Practice Circles for anyone looking for no rules and to get practice in for a real game jam. There we can help you further along together at your own pace as you learn at the start.
Hope to see you participate soon! Good Luck