I think it has been well confirmed fact that people of Kapilvastu, and Lumbini at large, 2500 years ago spoke the language hitherto known as "Paali". I don't think so. As the Kapilwastu and its surroundings were part of Bharat Barsha at that time, no part of Bharat bharsha had similar language like "pali". The origin of Pali is supposed to be Tibetian family and in modern era the language is little bit similar to Thai and Burmis languages. As Buddha spent most of his time in modern India ( Bihar and Uttar pradesh) his language must be originated from Sanskirta. The main reason behind the Buddhist manuscripts were written in Pali language is that they had been documented only after Buddhism spread in South East Asia.