* English is part of the Indo-European language family. Within that family, it belongs to the Germanic branch.
* Sanskrit is also part of the Indo-European family, but it belongs to the Indo-Iranian branch.
* Persian (or Farsi) is a Indo-Iranian language, directly related to Sanskrit.
* Spanish is part of the Indo-European family, specifically the Romance branch.
Therefore, there isn't a single language that English, Sanskrit, Persian, and Spanish are all derived from. They all share a common ancestor, but it's a very ancient language that no longer exists.
This common ancestor, known as Proto-Indo-European, is thought to have been spoken somewhere around Europe or Asia Minor about 6,000 years ago. Over time, Proto-Indo-European diversified into different branches, eventually giving rise to the many Indo-European languages we know today.