Douglas Harper's Etymology Dictionary
parleyvoo
colloquial for "the French language," 1754, from French parlez-vous (français?) "do you speak (French?)" For parlez, see parley (n.).
Wiktionary
parleyvoo
n. (context dated humorous English) Someone who speaks a foreign language, especially French vb. (context intransitive dated humorous English) To speak a foreign language, especially French.
Usage examples of "parleyvoo".
Why, Sir, my eldest daughter threw up a situation as parlour-maid in London because her master and mistress pitched to parleyvooing whenever they wanted to talk secrets at table.
So it may be that Parleyvoo is nervous about strangers they can't peg down.