Both sentences are correct.
Usually, the word "a" invites the listener to imagine a new object and add that object to their mental picture. The word "the," on the other hand, invites the listener to search their mental picture for an object which is already there.
So first, let's think about the first sentence:
A protocol is the language that a computer uses to communicate with other computers on the network.
This sentence is telling you the following:
- Imagine that there is a computer.
- Think about the network that the computer is on. (The sentence is assuming that the computer is on only one network.)
- Think about the language that the computer uses to communicate with other computers on the network. (The sentence is assuming that the computer only uses one language.)
- The word "protocol" means the kind of language that you are now thinking about.
Now let's think about the second sentence:
A protocol is a language that computers use to communicate with other computers on a network.
This sentence is telling you the following:
- Imagine that there are some computers.
- Imagine that there is a network that the computers are on.
- Imagine that there is a language that the computers use to communicate with each other.
- The word "protocol" means the kind of language that you are now thinking about.
Both sentences are correct, and they mean practically the same thing.
(Actually, I think your sentence is a little bit better, because one computer uses many different protocols. The original sentence makes it sound like one computer uses only one protocol.)