I would learn "C". The language is small in terms of key words. It has all the tools you will need to whiteboard a solution. You can't be an expert in a month but you can get a handle on the basics relatively fast.

Stay with the basics and don't make the mistake of trying to learn everything. You can't and you won't in a month.

If you aren't given a specific language, I would pseudocode the solution. Once you get a handle on pseudocode writing you can use it for all your whiteboard tests.

https://towardsdatascience.com/pseudocode-101-an-introductio...