After upgrading to 1.0.0-preview.18 (Unity 2020.3.17f1) my textinputs no longer use the system keyboard layout, but rather use a broken english layout.
Shift does not work, Numpad does not work, hitting # on my german keyboard inserts a carriage return, and so on.
Reverting the changes in InputSystem/InputSystemKeyboardEventProcessor.cs fixes the issue.
I am using the new Input System (on 1.1.0-pre.6).
Is this an issue on my end with the input system?
It’s definitely an issue with UI Toolkit preview 18, major apologies for that! We had an issue where special characters like Tab and Enter would be ignored on MacOS (with Input System) and the fix for that ended up causing this super huge regression, which passed under the radar probably in part because of the narrow combination of packages + OS where it happened. Now we have a much larger regression on our hands! I’ll see if we can’t push a quick fix for it, seeing how it makes UI Toolkit preview 18 pretty much unusable with the Input System package. 
Good to hear, thank you for the answer!
For now I am back on preview 17, but looking forward to the patch
You could tick the “Fallback On IMGUI Keyboard Events” box on the InputSystemEventSystem in the meantime if you still want to experiment with preview 18, until we fix this.