The phrase "quite interesting" is correct and usable in written English. You can use it to describe something that is intriguing, novel, or otherwise noteworthy. For example, "I read an article about the history of the British monarchy that was quite interesting.".
That was quite interesting".
It is quite interesting.
So that was quite interesting!
"Being Canadian, it's quite interesting".
Would make quite interesting reading.
"But it was quite interesting".
This too is quite interesting.
Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.
Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com