Full-stack developers must use a wide range of programming languages to accomplish their goals; these can be grouped into front-end and back-end languages.
Back-end languages are typically used to store and manipulate data, manage user permissions, and do all of the complicated calculations required by the program. There are a wide variety of languages that can be classified as back-end, including: Ruby, Python, PHP, Java, Go, and Swift.
In addition to being comfortable with both front-end and back-end languages, a full-stack developer should also have an understanding of the technology that runs alongside their programs. This includes a rough understanding of the SQL database language and the HTTP communication protocol.
The requirements of a full-stack developer can seem daunting at first, but it is important to remember that it is a "jack of all trades, master of none" profession. A full-stack developer will find themselves using multiple languages on a daily basis, but they will also typically find themselves working alongside dedicated front-end or back-end developers who have a much deeper understanding of their chosen technologies.