Nothing was developed first, but the Vedas are eternal and were pronounced in Sanskrit which is then, therefore, also eternal language just like Vedas are eternal because the Vedas are in Sanskrit. This is a very simple point to understand.
So both are eternal, and when something is eternal it makes no sense to say that there was something before and something later because eternity implies that it has no beginning in time. Putting it in other words, when something has no beginning in time it makes no sense to speak of "before" and "later".
That the Vedas are eternal is mentioned in many places in the scriptures, here is an example from the Manu-smriti, chapter 12:
- "The Veda is the eternal eye of the manes, gods, and men"
- "The eternal lore of the Veda upholds all created beings"