Although the language is Sanskrit, it is very different from the classical or traditional Sanskrit of today. The Sanskrit is called Vedic Sanskrit (a form of Indo-Iranian language) and the script is closer to certain ancient Iranian scripts than to Devnagri script of the modern age.

Sanskrit. The scholars have dated the existance near about 4th Century BC. It was the time when vedic Sanskrit was predominant along with Pali-prakrit. Though use of pali-prakrit is dated somewhere around 2-3rd century BC(according scriptures from the times of king Ashoka), Chanakya, who must have studied in vedic sanskrit language.. So, i think it must be written in vedic sanskrit.

Suryakanta Sastri has written:

'A grammatical dictionary of Sanskrit (Vedic), with a complete index to Wackernagel's Altindische Grammatik and Macdonell's Vedic grammar' -- subject(s): Sanskrit language, Dictionaries, English

'Sanskrit-Hindi-English Dictionary =' -- subject(s): Sanskrit language, Dictionaries, Hindi, English

The Vedic culture stemmed from the Vedas. which are the oldest scriptures of Hinduism. Its chief impact on Indian history was the spread and growth of Sanskrit.