I haven't often seen the weight of a dish on menus, with the major exception of cuts of beef, but this proved a standard feature of restaurant menus in Wrocław. Half-a-duck would have two weights, presumably one including the bones, and the other without. Multi-part dishes might have three or even four weights listed, divided by slashes, presumably one for each part of the dish, the better to indicate proportionality.
The weighing of dishes went so far that we ended up having lunch at a self-service place which sold its really-quite-decent food by weight alone, tareing for the plate. It was a good compromise between all-you-can-eat buffet and the moderation of paying for quantity.
Surely all these restaurants would not have bothered listing weights were it not legally required, whether by local or national law. In any case, it was oddly more alien to me than the foods themselves, although helpful as well in trying not to order too large a dish from restaurants specializing in hearty meat-rich portions.
P.S. In the odd way the world sometimes worked, I ended up needing to make a reservation at a Polish restaurant in London while physically in Poland.