No, they are not interchangeable, even though they do mean the same thing. Always use "command," unless you are talking about the Ten Commandments or something similar. The Ten Commandments are found in the Bible. Specifically they are found in the Old Testament, in the book of Exodus. The Bible says that they were carved into stone tablets that God gave to Moses. Some of the Ten Commandments include "Honor your father and mother," "You shall not commit murder," and "You shall not steal." Although the real Ten Commandments are serious, you will sometimes see lighthearted references to them. For example, someone writing about software engineering might say "the Ten Commandments of software engineering are: 1) Thou shalt plan before coding, 2) Thou shalt comment thy code, 3) Thou shalt test all thy code, even the parts that are obviously correct..." Or "the Ten Commandments of Camping in Montana are: 1) thou shalt bring food that is easy to prepare, 2) thou shalt arrive before dusk, 3) thou shalt bring bear spray..."

Hi, I'm a native speaker who hasn't studied English formally, but here is my intuitive take on this distinction: -A command is an authoritative order. A general can command his army to attack or withdraw, for example. -A commandment is also an authoritative order, but I've almost exclusively heard and read it in the context of religion. It holds greater weight or cosmic significance and (for me at least) it implies there is no alternative. The Ten Commandments, for example, are a distillation of moral tenets. They are so important that they can be counted and are taught very early on as a foundation of faith. Cheers!