There's loads of ways. Dead keys is one, the compose key is another suitable one for multilingual folk. I can write ä ö ü ß pretty much automatically with the c

There's loads of ways. Dead keys is one, the compose key is another suitable one for multilingual folk. I can write ä ö ü ß pretty much automatically with the compose key on a US layout, as well as any é ë ê ü I might need in Dutch. In the Netherlands letters with diacritics don't even have their own key (Dutch computers tend to use US plus €).