What language(s) should I use for my backend? [closed]

When I do research on the backend, I always see that there are multiple parts to it, such as the database, application, server... and see recommendations for different languages such as SQL or python. But do I really need different languages for the backend? For example, can Django(or any other backend language/library) handle all the main parts of the backend? And is PHP worth learning in 2020...