Learn Serbian on Tandem

Tandem is a free platform with a simple premise: there is probably someone out there who is learning your native language, and whose native language you want to learn. So they put you in touch with them. When you sign up, Tandem will review your application to make sure their platform remains safe and that the person you get matched with is also a keen language learner.
Price: depends
Devices: Android, iPad, iPhone, and Web

Learn Serbian on Qlango

Qlango is a language learning app that promises to help you learn a new language in 5-minute increments, as each lesson lasts that long. It offers quick and easy lessons, along with interactive games, flashcards, and chatbots to help you learn vocabulary and grammar.
Price: Free
Devices: Android, and iPhone