"Interested in" is the correct prepositional phrase to show that someone likes or is curious about something. "On", "with", and "by" are not used in this context.
She is interested in learning new languages. / അവൾക്ക് പുതിയ ഭാഷകൾ പഠിക്കാൻ താൽപ്പര്യമുണ്ട്.