Use case
As a software developer I prefer using keyboards with US-layout. Mainly because special characters (like []{}\|<>) are more easy to write/reach – some even without a modifier.
But, for email and stuff, I still rely on German Umlauts.
Solution
So this is what I did do use German Umlauts even with US-keyboard.
Create a .Xmodmap file in your home directory
nano .Xmodmap
And add following lines:
keycode 108 = Mode_switch keycode 26 = e E EuroSign EuroSign e E keycode 30 = u U udiaeresis Udiaeresis u U keycode 32 = o O odiaeresis Odiaeresis o O keycode 38 = a A adiaeresis Adiaeresis a A keycode 39 = s S ssharp ssharp s S
Conclusion
That’s it. Now you can access both the lower- and upper-case Umlauts by using the right ALT key.
If you don’t want to restart your X-server to see the changes you can re-import the xmodmap settings as follow (again in your home directory):
xmodmap .Xmodmap
I did it utilizing Ubuntu 15.10 with a UK console and EN-GB console. I don’t comprehend what a Compose key is so didn’t deliberately utilize one. Rather the accompanying keystrokes put umlauts over ü and ö.