Arabic speakers have a comparable system:

https://en.wikipedia.org/wiki/Arabic_chat_alphabet#Compariso...

in German there is an official convention for the non us-ascii letters, AFIK it predates computers and is rooted in germanic dialects developing differently and later stuff like non-german specific type writers, printing press machines, etc.

ä => ae

ö => oe

ü => ue

ß => ss

but one gotcha is that it's a fuzzy one way trip, some words, especially city names, can have a ae,oe,ue in their correct native spelling. Worse ss is a normal language building block in German which is pronounced differently then ß so writing it as ss is quite confusing for anyone which doesn't happen to know that it's correct spelling is with ß. To top that of some cases of ß spelling have officially changed to ss spelling over time due to people anyway pronouncing it more like a ss and getting it wrong all the time. And to some degree ß is semi-official abandoned by now.