What Mark Kramer said about saying "Learn on!" is true, but I really agree that it would be a bit odd to say this generally.
"Learn on" is not a construction I've really heard outside of the "on" denoting the thing you are using to learn. For example... "Languages are easy to learn on the computer."
If you wanted to denote ongoing learning, you would just say, "I'm learning English." or for continuing the action into the future, "I'm going to keep/continue learning English."
* You could also say, "I'll go on learning... (until I understand/ until there is nothing more to learn/ until I die... etc.)."