If we look at the few countries that currently use mph for speed limits then certainly Wales will be the first to do so. However many countries using km/h, where the equivalent is 30km/h, have this as a norm for most urban/village roads. Spain has taken a step similar to Wales when it set a national 30km/h limit for any urban/village roads with a lane in each direction to 30km/h in 2021. This is reduced to 20km/h (12mph) if there is no pavement.
Wales will be the first nation within the UK to set a national 20mph limit, however Scotland plans for 20mph to be the norm by 2025.
Already 18 million people in England live in local authorities who have rejected 30mph as an urban/village norm and have set 20mph for most such roads. This includes all the inner London boroughs, most of the large cities and more rural counties such as Lancashire and Cornwall. See our full list of 20mph places.