What is the best language architecture to create chat apps like WhatsApp which consume less data?

The language used to build a chat application like WhatsApp are:

Ejabberd - WhatsApp uses the open source jabber resembles to enhance the operational efficiency.

Erlang - The programming language used to script the real-time chat app system

XMPP - XMPP is the backbone that makes it possible to send real-time messages, online presence indicators, XML routing features and much more.

PHP - Open-source general-purpose scripting language best fit for web development.

FreeBSD - Advanced computer operating system handpicked for everything from desktops to tiny devices.

Yaws - An Erlang-based web server that can also run as a standalone web server

Lighttpd - Another web server that is highly secure, swift, flexible and compliant with web server standards.

Contus Fly is a readymade solution which helps to build a real-time chat app like WhatsApp instantly with advanced features.

Its little bit complicated task but there are many services available that can help you building app like whatsapp, you can google about it.

but still if you want to do all task by yourself you need to learn multipal languages. here are some

PYTHON, PHP, JAVA, RUBY

after learning these language you will get enough knowledge how you can start work on your project.