I know that the common stereotypes of English (just generalizations) are that they are stuffy, proper, etc. French stereotypes are that of a romantic (the French language itself lends itself to fluidity, and the language of love, etc) so I would imagine that's what Mary is referring to. After all, she continues to say:

"All that English abilities can do has been tried already. I have three very particular friends who have been all dying for him in their turn; and the pains which they, their mothers (very clever women), as well as my dear aunt and myself, have taken to reason, coax, or trick him into marrying, is inconceivable! He is the most horrible flirt that can be imagined. If your Miss Bertrams do not like to have their hearts broke, let them avoid Henry." (Ch. 4)