Alt-shift changes the keyboard language if you have multiple language installed in Windows 10. If you sometimes wonder why your computer's language mysterious changed -- for me, it turns out this is the shortcut that's causing the problem. I use OneNote heavily and I keep hitting alt-shift by accident because I use the alt+shift+arrow key shortcut to move note items around. When I changed my mind about moving an item, I would often hit alt-shift, changed my mind, and release the keys without hitting an arrow key. It took me a few years before realizing that is the reason why my language sometimes "just changed by itself".
To fix it -- and by that I mean getting rid of this language shortcut -- follow the directions here: https://superuser.com/questions/698037/can-i-disable-the-altshift-shortcut-to-change-language-in-windows-8-1-or-win#
Basically, you run the following command to open the dialog box option for the language shortcuts, and disable the shortcut. I have to use this because with each windows 10 update Microsoft likes to moves its options around, and it's hard to actually find this option otherwise.
"C:\Windows\system32\rundll32.exe" Shell32.dll,Control_RunDLL input.dll,,{C07337D3-DB2C-4D0B-9A93-B722A6C106E2}{HOTKEYS}