How do I access special characters on Apple's Smart Keyboard?
You should be able to use the standard Apple option key shortcuts, such as ⌥ + e, then e produces é. One list is here.
For the german "Umlaut", it is:
⌥ + u (for Umlaut) and then the letter (like a, u or o)
So: ⌥ + u and then a produces “ä”