Umlauts on Sierra on English keyboard with Alt combo not working anymore for all
The default key combo in Sierra and older OS Xces on UK/US (physical and logical) keyboards to enter Umlauts is:

altu + a: ä
altu + o: ö
altu + u: ü  

You probably had a key modifier (like Karabiner) installed in your previous systems and remapped the key combo to get your non-default behavior.

At least Karabiner doesn't work in Sierra currently (as of Oct 2016) and you have to either wait for an update and/or use the old key combo again.