In Vinaya, Cullavagga (Vin. II, PTS, p. 139), the Buddha advises bhikkhus not to use Vedic language (Chanda; i.e. Vedic Sanskrit) for the Buddha’s language/teachings (buddhavacana ), but use your own language (sakāya niruttiyā ‘based on your own language’) for the Buddha’s teachings.
As a result, there are now different textual languages for the teachings and stories in Early Buddhism.
Pali, literally ‘text’, is based on a dialect (a Prakrit) from the region of Ujjeni/Ujjayani/Ujjain, capital of Avanti, in western India. According to the Sinhalese Buddhist tradition, Mahinda and Saṅghamittā, who preached Buddhism in modern Sri Lanka, were born in Ujjeni.
As stated previously, early Buddhist texts, such as the principal four Nikayas/Agamas, were in fact not established at once in complete structure (form) and content at the first Buddhist council.
According to Ven. YinShun, the principal four Nikayas/Agamas were gradually developed and expanded from Samyutta-nikaya (SN)/Samyukta-agama (SA). https://discourse.suttacentral.net/t/ve … hism/22540
The extant SA and SN, and also other Agamas/Nikayas, are sectarian texts. One can seek an understanding of early Buddhist teachings by studying them comparatively.
Nevertheless, the major early Buddhist teachings are shared in common in the extant SA and SN; e.g. see the following book by Choong Mun-keat:
The Fundamental Teachings of Early Buddhism: A Comparative Study Based on the Sūtrāṅga portion of the Pāli Saṃyutta-Nikāya and the Chinese Saṃyuktāgama (Series: Beitrage zur Indologie Band 32; Harrassowitz Verlag, Wiesbaden, 2000).
The particular collection of the Pali SN and the Chinese SA is mainly about knowing and seeing the four noble truths, the notion of anicca, dukkha, suñña (empty), anatta, and the middle way, which all are the core teachings of Early Buddhism.
So, Early Buddhism (regarding EBTs), historically, may have two phases: (1) Samyutta/Samyukta Buddhism (i.e. based on the so-called ‘Connected Discourses’ 相應教, Saṃyukta-kathā) and (2) Nikayas/Agamas Buddhism (i.e. mainly based on the principal four Nikayas/Agamas).
However, as mentioned above, the extant four Nikayas/Agamas are sectarian texts. One can seek an understanding of early Buddhist teachings by studying them comparatively.