Most early Jain sutras were composed in Jain Prakrits—especially Ardhamāgadhī Prakrit.

• Shvetāmbara tradition: the Āgamas (canonical sutras) are preserved mainly in Ardhamāgadhī Prakrit.
• Digambara tradition: they hold the original Āgamas were lost; their authoritative works and sutras are in various Jain Prakrits (Ardhamāgadhī, Śaurasenī, Māhārāṣṭrī) and later in Sanskrit.
• Some key Jain sutras (like Tattvārtha Sūtra) are in Sanskrit.

So, not all are in one language, but the foundational layer is chiefly Ardhamāgadhī Prakrit, with important later texts in other Prakrits and Sanskrit.