What has happened, when sporadically the keyboard language in Win 10 has switched without purpose?Probably you accidentally hit ctrl / shift, which is the default setting in Win 10 for switching the keyboard language and a lot of people are not aware of it and they think it's a bug of Windows. But there's hope because you can disable this function here:
Goto "Control panel / Language / Advanced settings / change language bar hotkeys / change key sequence / Switch keyboard layout" and change it to not assigned.
If this doesn't work, remove all the unwanted keyboard languages - even when it's English (e.g. for foreign people).