Ok, this is kind of making me crazy.
I've installed Steam on plenty of different OS and PC over the years, some with system language in Italian, some other in English.
Occasionally, in the Steam client chat (but IIRC even in some other place I can't remember right now), when I focus on the input field, a little "IT" tooltip appears in the top right corner:
You probably have two (or more) typing languages/types of keyboard installed on Windows. I, for instance, have Brazilian Portuguese and Japanese on my computer, so that's there.
It shoudl also be on windows bar, by your docked applications, by the clock.
Left Shift+Left Alt will cycle through them on any focused application.
I'd say you'll have to remove the different languages/keyboard inputs that you don't use in Windows.