Keyboard language keeps changing in Windows 10
In Windows 10, by default, pressing CTRL+SHIFT (or for some ALT+SHIFT - thanks madmenyo ) will cycle through any keyboard layouts that you might have mapped and it’s surprisingly easy to do this by mistake.
If you keep pressing CTRL+SHIFT (or whatever you might have changed it to) then soon you should get back to the correct setting. (alternatively reboot which is what I did first time 
)
(Updated Aug 2019) You can change/disable this by
> Settings > search for 'typing' > Advanced keyboard settings > Language
> Bar options > Advanced Key Settings (tab) > Change Key Sequence
Be warned, the above doesn’t always work - Restarts and Sleep mode can both change keyboard default (usually to US) - I’ve found no cast-iron solution though creating a new profile can help, though not a particularly satisfactory answer IMHO.
In an emergency
WIN+R
osk
to bring up the On Screen Keyboard might help temporarily.
Also note that it’s possible to disable this so that no key combination will change the language - change the keys to “Not Assigned” - see answer below from Mort for more info