Telegram's Android client is listed as written in Java. Java was the dominant programming language for Android applications for many years, and Telegram's official Android client has historically used it for its user interface and application logic.
Telegram's Android codebase is open source, allowing developers to inspect how the client handles conversations, media, notifications, settings, and other features. The app also depends on native components and shared libraries for performance-sensitive work, so an infobox language label should be read as a summary rather than a claim that every line uses only one language.
Kotlin is now a major Android language and is used widely across the Android ecosystem, which makes it a tempting answer. Swift is associated primarily with Apple's iOS and macOS development, while Python is not the language listed for Telegram's Android client. The question follows the language designation in the reference entry, not every implementation detail of the evolving source tree.