Given that you are familliar with Javascript already. You can pick up nodejs easily. Using nodejs for a while will give you an idea of what backend development's daily life look like.

Once you understand the common tasks that are involved in backend development, it will be easier to switch programming languages. Python is a good choice for example, it is very versatile and has been used to program various backend server. It is also being used extensively in scientific computing, machine learning and etc.