While living near Dover, we did regular gigs at a cafe in Boulogne old town on le quatorze & other times.
A handy piano accordionist with us (Oscar Simmons of Gravesend) knew such as 'J'attendrai' & a rude Canadian words to La Marseillaise'- we didn't sing & just played music mainly.
Passing English daytrippers would whisper
'I'm sure they're not French, you know'

- we used to get away with it till we met them while busking the duty-free queue on the ferry back- mind you Oscar's fluent French, moustache and striped shirt helped- although the string of onions over his shoulder were a nice touch...   happy days!!