[usr1106]

There is an ICAO recommendation. However, it is not unambiguous and of course it's not legally binding. So in the end every country decides what they do. (Possibly there are more multinational agreements e. g. inside EU, but I doubt there is anything truly worldwide.)

https://www.icao.int/publications/Documents/9303_p3_cons_en....

Ü is written as UE, UXX or U

Ű is written as U

According to https://en.wikipedia.org/wiki/Machine-readable_passport#Name... Hungary uses UE for Ü, but there is no reference given. According to the same article Russia uses even 2 different transliteration systems depending on the type of document.