Will other cities in Belgium also introduce a LEZ?
Yes: there is a low emission zone in the city of Antwerp since 1/02/2017. Mechelen and Ghent will also introduce a LEZ from 2020.
Note: The access criteria of the LEZ in Flanders are not exactly the same as the LEZ in Brussels. Relevant information about the LEZ in Flanders can be found here.
In December 2018, the Walloon Government passed a pre-project law that will make it possible for Walloon municipalities to implement permanent or occasionnal low emission zones. Note that these zones will not function equally as the Brussels zone of the zones in Flanders. More info here (in French): http://diantonio.wallonie.be/home/presse--actualites/publications/publication-presse--actualites-6.publicationfull.html
Are there LEZs in other countries?
Yes: many European countries have introduced LEZs in their major cities. Most major German cities have one.
They also exist in the Netherlands and Italy.
London has a LEZ and Paris has recently established a sticker system.
All information regarding LEZs in Europe can be found here.