My trick is to setup both German and English keyboard and you can switch between the keyboard layouts while typing by pressing "alt" + "shift"
1. Go to Start -> Settings -> Control Panel -> Regional Settings and Language Options -> Languages tab -> Details
2. Your default is probably English US or Irish or UK
3. Click Add to choose "German (German)" as your language and select your keyboard layout "German"
4. Then check "Language Bar" and make sure "Show the language bar on the desktop" is checked
5. You can then see which input is default on your task bar by the clock. And switch input either by clicking on it or using "alt" + "shift" combination.
The downside is that you should already know were the ä ü ß etc are on a German keyboard or else it's guesswork. Get stickers?
My keyboard is german but I use English as default as I'm handy with touch typing, so it is easy for me to switch between the too.