Will special characters such as ü, ö ß, ä, etc. work with KDP and CreateSpace?

Accented characters aren't a problem with either KDP or CreateSpace.  For CreateSpace, make sure that the font you are using is embedded into the PDF and it will definitely print just fine. For Kindle, make sure you test your book using Kindle Previewer because sometimes they can get mangled; I've actually written myself a little script that will replace accented or special characters with their HTML equivalents (it changes 119 special characters, so a u with a diaereses – ü – would actually be written ü instead in my document, for example.) I'm still trying to work out when kindlegen will mangle the letters and when it won't; usually something that comes straight out of WordCrusher is okay, but if I edit it in my favorite text editor on my Mac then it almost always mangles the special characters.