I would use the language you are most familiar with so that you can code the quickest. Then spend time checking your code thoroughly and optimising it as much as possible. Also prepare, if you can, by finding out how the compiler you will be using will optimise your code so you can make the best of the compiler.

As in all situations like this don't be tempted to rush - take it slowly and carefully.

Good luck - let us know what your decision was and how you performed.

Alan