I thought that you were allowed to choose the letters that didn’t refer to the municipality as long as they weren’t already taken. Hence why in Mercedes publicity photos the license plate always reads S (for Stuttgart) GO [some numbers].
In Europe I think the big difference is that you don’t need new plates when a car changes owner. If you buy a used car it keeps its number unless you choose to buy a number from someone else (originally issued to another car) and put it on.
Here in the UK, they have to fit a defined system but people still pay quite a lot for ones that form words or contain their initials. For instance, the one Vauxhall uses for its ad photos is DE51 RED.
