When a non-native writer writes a paragraph which is difficult to understand because of some errors and unnatural use of the language, could we call this kind of language for example 'a bit jargon' or 'jargon' is not used this way ?
No. Jargon is not "incorrect". It may be unnatural use of expressions that mean something else in other circumstances, but it is not wrong use. It's special use.
Jargon usually uses specialist words that fit within given narrow domain of expertise, but it frequently borrows words from "mainstream" giving them new meanings, and this could cause a lot of confusion.
When you fork a thread to bang bits over the bus, it has nothing to do with skewering a sewing material with an eating utensil in order to hammer small pieces above a public transport vehicle. It's jargon for making a simultaneously running duplicate of a certain program running in the computer memory, and assigning the duplicate a task of sending zeros or ones one after another in specific sentence through a connector that connects multiple devices. But as you can see, telling this in non-jargon way is overly verbose and pointlessly delves into details any professional in the field doesn't need repeated.
Jargon is never used to refer to unnatural or incorrect use of language by non-native speakers. It refers to words and phrases that are very specialized to some area of business, science, or technology. "I can't read this because it's full of computer jargon." "Can't we have a conversation without you using medical jargon?"