Question
What is the language(lipi) in which the Vedas are written?
Answer
संस्कृतम् (Sanskritam)
Any language can have 2 components - sound and sight - the words we hear and the script we write.
The sounds of Vedas are eternal (literally exist infinite time before and infinite time after). They are not made by man, rather they just exist in nature as the breath of Brahmam, and are perceived by rishis during deep state of tapas. The language that the sounds of Vedas exist in is called Sanskrit and were formalized in grammar sutras by Panini, and then by Patanjali.
Until ~5000 years ago, when Kali Yuga started in 3102 B.C, Vedas and other Sanskrit literature were mostly recited and memorized by generation-to-generation. Rishis foresaw that human intellect would decrease as Kali progressed, hence they wrote down the sounds of Vedas in a representative script. This script, of course, is man-made, hence why it keeps changing over time.
The history of script changes is detailed/complex. Most Indian languages fall under Brahmic script. Devanagari is currently the script used to represent Sanskrit (and Hindi) sounds.
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