"I do not understand the language."
Use vI- when you're doing something (or, in this case, not doing something) to something else. Hol vIyajbe' I do not understand the language. What you're doing is not understanding, and the thing you're doing it to is the language. The thing you're doing it to is called the object, sometimes the direct object.
Use jI- when there is no object. jIyajbe' I don't understand. You're not naming a thing you don't understand; you're just saying that in general you don't understand.
An explanation is given in the Tips & Notes. It’s a lot of new information and some details are bound to slip through the memory. But since Duolingo has hidden the Tips & Notes I want to make sure you know about them and where to find them. If you have not been reading the Tips & Notes, I would like to ask that you review those so we don’t have to continuously repeat the information that we have explained there.
If you are doing the course on iOS or Android, you cannot currently access the Tips & Notes through the app. To access the Tips & Notes, you will have to access the course using a web browser at https://www.duolingo.com/. You can still do it on your mobile device, but you will have to use the web browser instead of the app (or you can do it from a computer). When you click on a Skill, it will expand to reveal a Start button, a key, and a light bulb.
If you click on the light bulb it will reveal the Tips & Notes and give you a detailed explanation of the grammar that is introduced in that Skill. If you have questions after reading the Tips & Notes for any Skills, then please return to the forum to ask your question, explaining what you didn’t understand or what seems contradictory to you.
In addition to what DavidTrimb3 and jdmcowan have said, it is not jl-, vl- with a small L; it's jI-, vI- with a capital i.
You should be able to see a little curl at the bottom of a small L l which is missing at the bottom of a capital i I -- pay attention to this slight difference!