[usr1106]

> Diacritics are usually stripped

Not in all cases. In Germany and Finland (maybe all EU passports???) ä is spelled ae, ö is spelled oe in the machine readable part (umlauts shown in the "human-readable" part). This is important to know when you need a visa.

For Germans this is not a big problem because it has been like this forever if the umlaut is not available for technical reasons. For Finns this is a problem, because this "transcription" is completely unknown in Finnish. For a couple of weeks now it has been possible to get an electronic visa for Russia on the internet. Reportedly many Finns with an ä in their name (that's not uncommon) dropped the dots when applying for their visa, because an ä is not accepted. At the border they were not allowed to enter, because the machine-readable part of the passport has ae instead.

[bonoboTP]

Good point, I don't know any Hungarians with ü or ö in their name, just á and é.

I do wonder what happens to ű and ő though.

[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.