If your goal is to learn the most about Jainism, the best language to start with is Prakrit, especially Ardhamagadhi Prakrit.

Why Prakrit?

• Many important Jain scriptures, especially in the Shwetambar tradition, are in Ardhamagadhi Prakrit.
• It is closely connected to the original teachings of the Tirthankars and early Jain literature.
• Learning Prakrit gives you access to the authentic scriptural style and terminology.

Also very useful:

• Sanskrit: Many later Jain philosophical texts, commentaries, and stotras are in Sanskrit.
• Gujarati / Hindi / English: These are helpful for modern study, translations, and explanations, but they are not the original scriptural languages.

Best order for study:

1. Ardhamagadhi Prakrit
2. Sanskrit
3. Then a modern language like Hindi or English for commentaries and explanations

Simple conclusion:

If you want the deepest direct access to Jain scripture, choose Ardhamagadhi Prakrit. If you want the broadest scholarly understanding, learn Prakrit + Sanskrit.