https://www.quora.com/What-language-did-Gautama-Buddha-speak-according-to-Indian-mythology-and-why-is-it-significant

The Buddha spoke in a language called Magadhi Prakrit.

Magadhi Prakrit is the spoken language of
the ancient Magadha kingdom, one of the 16 city-state kingdoms  at the
time, located in the eastern Indian subcontinent, in a region around
modern-day Bihār,
and spanning what is now eastern India, Bangladesh, and Nepal. The
first Magadha king is Bimbisara (558 BC –491 BC), during whose reign the
Buddha attained enlightenment. Both king Bimbisara and his successor
son Ajatashatru, were mentioned in several Buddhist Sutras, being lay disciplines, great friends and protectors of the Buddha

Magadha empire, ~500 BCE

The Magadha kingdom later became part of the Mauryan Empire, one of the world’s largest empires in its time, and the largest ever in the Indian subcontinent.

Mauryan Empire, 265 BCE

Magadhi Prakrit is the official language of the Mauryan court. Its emperor “Ashoka
the Great” (ruled 273- 232 BCE) united continental India. During the
war to conquer Kalinga, the last Southern part of India not subject to
his rule, he personally witnessed the devastation that caused hundred of
thousands of deaths, and began feeling remorse. Although the annexation
of Kalinga was completed, Ashoka embraced the teachings of Buddhism, and renounced war and violence. He sent out missionaries to travel around Asia - his son Mahinda  and daughter Sanghamitra,  who established Buddhism in Ceylon (now Sri Lanka) - and spread  Buddhism to other countries.

Stone lion of Ashoka, later became symbol of modern India

Magadhi Prakrit is predominantly the language by which Emperor Ashoka’s edicts  were composed in. These edicts were carved on stone pillars placed throughout the empire.

The inscriptions on the pillars described edicts about morality based on Buddhist tenets.

Ashoka Pillar at Feroze Shah Kotla, Delhi, written in Magadhi, Brami and Urdu

Geographically,
the Buddha taught in Magadha, but the four most important places in his
life are all outside of it. It is likely that he taught in several
closely related dialects of Middle Indo-Aryan, which had a high degree
of mutual intelligibility.

http://www.indicstudies.us/Archives/Linguistics/Brahmi.html

The Brahmi script was the
ancestor of all

South Asian writing
systems. In
addition, many East and Southeast Asian scripts,
such as Burmese, Thai,

Tibetan, and even
Japanese to a very small extent (vowel order),
were also ultimately derived from the Brahmi
script. Thus the Brahmi script was the Indian
equivalent of the Greek script that gave arise
to a host of different systems. You can take a
look at


the evolution of Indian scripts,
or

the evolution of Southeast Asian scripts.
Both of these pages are located at the very
impressive site

Languages and Scripts of India.
You can also take a look at

Asoka’s edict at Girnar,
inscribed in the Brahmi script.

Related links:

• 
  
  Languages and Scripts of
  India
  
• 
  
  Eden’s Page: Scripts of all
  of Asia
  
•