A friend gifted me a laptop with US layout. One day I figured I really needed those German characters. I did the same as @okiloki@feddit.de, except I ended up picking the US-International layout, and removed the old layout since I didn’t need it anymore.

Put your keyboard to “US international with dead keys”, then use quote (") and the letter, you’ll get an umlaut.

If you talk about a physical keyboard that is plugged into your steam deck: have a look at eurkey

How are the possibilities to change the on screen keyboard?

Steam Decks run Linux. (The specific DE is KDE Plasma I think.) So you can find answers by searching for “Linux” if searching for “Steam Deck” doesn’t get results.

One way is to enable the “Compose” key which lets you enter special characters or sequences by typing switches of more commonly-available characters. I think the Steamdeck OS has a setting for this; but I don’t have one so I can’t check.

For letters with umlauts you press (and release) Compose, then type a double quote (need to hold shift for this part), then type a vowel.

For reference Wikipedia has a list of Common Compose Combinations

Alternatively if you can map an AltGr key I’ve read you can type umlauts by typing AltGr+[ and then typing a vowel. There might be a setting for this too.

Enter desktop mode. Go to the system settings -> Input devices -> Keyboards and click on the Layouts tab in the top. Check off the box next to Configure layouts then click the Add button to continue. Search for German, click on Ok and then on apply. Hope this helps!

my guess is that you have a custom keyboard with qmk firmware (why else use ANSI layout?). so pick a mofifier key in VIA and make it switch the keyboard to a different layer, then maybe set the A O U and maybe S keys to Ä Ö Ü and ß. if you’re not running QMK, just buy a DE layout keyboard for god’s sake.