ooh...
Well, everytime I heard of the word, it basically meant couples meeting other couples to swap sexual partners, for example in "swing clubs". Apparently, "swingers" give the activity a much broader definition, though. You might want to check http://en.wikipedia.org/wiki/Swinging to learn more.
In French, I guess it's a variant of "echangisme". I can't speak Greek, but with this context, I'm sure someone will be able to help you with the translation