his comparison of English and French is more to Frank's general behavior in not visiting the Westons, rather than his perception of Frank's view of Mrs. Weston's social position. "Amiable" has roots in French, and while Emma is applying the English meaning to the word, Mr. Knightley seems to be refering to it's French origins and views Frank Churchill's behavior as more French than English. Note the specification of "English delicacy". The negative comparison to the French is appropriate, after all, the French were not very popular in England at this time.

"No, Emma, your amiable young man can be amiable only in French, not in English. He may be very "amiable," have very good manners, and be very agreeable; but he can have no English delicacy towards the feelings of other people: nothing really amiable about him." (Chapter 18)

I do beg pardon for barging in to an already established Group Read, without proper introduction. I hope you will forgive my lack of propriety. ;D