If I can be forgiven for interrupting a T.A.W. thread, many homeschooling families see a benefit in teaching an inflected language like Latin or Ancient Greek. That teaches concepts (like case) that will make a second or third language easier.
Also, as has already been said, there is a big overlap between Greek and Latin vocab and technical English vocab. For example, the English words thaumaturgy (wonder-working) and gynaecology (study of women) are essentially Greek. So in teaching Greek or Latin, you are also teaching English (also, the Greek alphabet is needed for mathematics).
However, I would expect that, for Orthodox, the Greek New Testament, the Septuagint, and the Greek Fathers would tip the balance towards Ancient Greek.
That's assuming that you don't have a strong family or church connection to Russian. If you do, Russian as a 2nd language would make sense.