What is the language in which Vedas are written? Currently, they are written in Sanskrit language but what was their first language. Devnagri is developed by humans and Vedas are the language of gods so what is their initial writing language? Were they in any other language? Specifically, what was their lipi or script i.e. written form called?
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.
All Vedas are written in Sanskrit. Sanskrit is believed to be the language of the Devas. Hence it's script is called Devanagari(of the city of gods).
Initially they were remembered in the mind only, but with the passage of time, human memory became weakened so they had to be put in writing. Even this was considered sacrilege
Vedas started with just भाषा(language). लिपि(script) came long after that. The trend of writing in देवनागरी(devanaagari) script is very recent.
First - about the Veda: Vedas are known as Shrutis(something that was heard). It was preserved for generations as an oral tradition (कण्ठस्थ).
Second - the language: The language of the Vedas was known by many names such as ब्राह्मी(braahmi) भारती(bhaarati) भाषा(bhasha) सरस्वती(saraswati) etc. Given the antiquity the Vedas संस्कृत(Sanskrit) is a very modern nomenclature. The great Panini in his अष्टाध्यायी, considered to be the seminal treatise on Sanskrit grammar, has not used the word Sanskrit.
Third - the script: Vedas were preserved as an oral tradition hence there was no script associated with it. Since the time they learned how to write, it has been written in many scripts. ब्राह्मी(braahmi), नागरी(naagari), नन्दिनागरी(nandinaagari), देवनागरी(devanaagari). More recently (last couple century or so)every region wrote it in its own script. So you will find Vedas and other Sanskrit literature written in Odia, Bengali, Tamil, Telugu, Kannada, Malayalam, Assamese and many more. In fact this diversity of scripts is the prime reason of popularity of देवनागरी in recent times. देवनागरी has become the primary script now.