If you use a Mac with a US keyboard but still write German regularly, the USGerman keyboard layout is a small quality-of-life fix. It keeps the normal US layout and adds German umlauts and ß behind the Option key, so you do not have to switch to a full German keyboard layout or use macOS’ long-press character picker.

After downloading and unzipping the layout, copy Roman.bundle into the system keyboard layouts folder:

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sudo mkdir -p "/Library/Keyboard Layouts"
sudo cp -R Roman.bundle "/Library/Keyboard Layouts/"

Do not put it into ~/Library/Keyboard Layouts on modern macOS, because it can cause trouble when entering administrator passwords.

Then open System Settings, go to Keyboard, edit Input Sources, add the layout under Others, and select U.S. With Umlauts via Option Key. Log out and back in once so the layout works everywhere.

The shortcuts are simple:

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Option-a  → ä
Option-o  → ö
Option-u  → ü
Option-s  → ß

The uppercase variants work with Shift, so Shift-Option-A gives Ä, Shift-Option-O gives Ö, and Shift-Option-U gives Ü.

Resources: