Yes, I agree. Although there is some semantic overlap, skills are more about practical, hands on kind of knowledge while "Kenntnisse" are more theoretical in nature. "Fertigkeiten, Fähigkeiten, Geschick" are better fits for 'skills'. I can have a lot of Kenntnisse about singing without getting a single note right in a performance. You probably wouldn't call me a skilled singer then... I have no issue with translating 'language skills' to "Sprachkenntnisse", though. As I said, there surely is some semantic overlap.
So what would be the plural of kenntnis? "The skills" is not an acceptable translation, apparently.
I don't think "understanding" is a good translation of Kenntnis.
The main use, I think, is Kenntnis haben von etwas "to know about something" i.e. to have information about something that has occurred, to be aware of something, and similarly mit/ohne die Kenntnis von X "with(out) X's knowledge".