Thomas Jeffreys, linguistic at large :-), says:

[Re Japanese borrowings form Chinese]I know. It doesn't count. 
The majority of the lexicon of any language is
very fluid. Languages can (and do) borrow like crazy. The majority of
English words are derived from French and Latin, but that doesn't make
English a Romance language. 

But aren't English and Romance words still cognates of each other if they
share the same root?  E.g. celestial/caelonis (or whatever; it's a long
time since I did Latin, and I don't get much practive :-) ).

I know that when I compared notes with someone who was learning Chinese,
we found that a lot of Japanese words are pronounced essentially the
same as their Japanese counterparts.  Wouldn't these be cognates?

Gavin