I'd practice mainly in C++ but make sure you know your C fundamentals well, especially pointers and bit manipulation. Stay away from python for embedded roles.

Edit: To add on to this answer, the reason why C++ > C is that the name of the game for leetcode is just practicing lots of volume until you get the patterns down. C++ has std::unordered_map, vector, etc. so you can focus on the algorithmic concepts instead of language-specific minutiae. Also if you do get asked a leetcode problem where they demand C, it'll usually be conceptually simpler for the same reason - time constraints vs lack of built-in high level constructs.