"I do not understand what you are saying."
Translation:Я не понимаю, что ты говоришь.
If I want to insert an extra pronoun, should it be 'to' or 'togo'? (Я не понимаю то/того, что ты говоришь.)
Yes, "Скажешь" would mean "(you) will say".
"Сказать" is a perfective verb, which means it can't be used in the present tense. If you conjugate it like a present tense verb, it becomes future tense.
How would the Russian differ from the English, "I don't understand - what are you saying?" I thought the correct sentence would rather be "я не понимаю то, что ты говоришь"