A friend told me that Tony Hancock claimed that the double-entendre
was unique to British comedy. Surely this cannot be true. Could you
please find Hancock's original quote and examples of double-entendres
in other languages / countries. Thanks.

That is BS, of course, as the French expression itself suggests.

Here is a German one:  We received a wedding gift of two framed
pictures of birds, the giver remarking slyly as he presented them: 
"Aus Liebe zu Vögeln."
This translates:  Out of love for birds, but "Vögeln" is not just the
dative plural for birds but also a verb for sexual intercourse.

Of course there are many others, but they will only come to me when I
have posted this.  When my wife was much younger, she heard a very
good joke, and even remembered it, retelling it once, to everyone's
amusement, and then started to tell to different company, when
suddenly she finally recognized the double-entendre that everyone else
had been laughing about.