The US standard sizes are 30" and 36" although there are narrower units available. I was looking at some “European wall ovens” and noticed they were also 30". Are there metric versions also?
Ovens here in South Africa (not Europe, but metric) mostly come in 60cm and 90cm, and occasionally 70cm, widths.
60cm is the standard width in Norway, and (I checked at ikea.se) in Sweden built ins are 59.5 cm wide, for a 60cm gap. Ikea is good for this sort of thing because they have stores in most countries and use the same names. Which is how I now know they sell the same width oven in Germany as well.
Of course there could be countries where Ikea has one standard and everyone else has a different one.
60 cm is EN/EU standard. Switzerland uses 55 cm as standard, but it’s possible to get 60cm, which is what we did when we updated our kitchen. It’s better to be tied to a standard width which is followed by more countries.
Yes, 60 cm (23.6") is the standard for kitchen appliances in the uk.
Though a small number of ovens are 50 cm. Like ours.
j
ETA - In the UK
No room for a turkey then… 
Wedge it in diagonally.
Man, it’s a pain in the ass when you need to replace an oven like that. There are 50 cm models in the UK, but not many to choose from.
j
Our ovens (and dishwashers, washing machines, and dryers) fit into kitchen units 600 mm wide. I guess that means the appliance itself is slightly less than 600 mm as it is not an interference fit.