It depends on the country and/or the city.

In France, it is legal:

* in all "30 km/h zones" (which are a bit different from a single street with a 30 km/h limit), unless specifically disallowed in a specific street;

* in all "'meeting' zones" (where the speed limit is 20 km/h and pedestrians have special rights);

* and of course anywhere where it is specifically allowed.