password fails because keyboard language changed

Note to community. Last night I installed a Czech keyboard in order to write a few phrases in Czech, a language that requires letters that don't appear in the English language. I switched back to English and continued working for an hour, then shut down my computer. The next morning, my MacBook Pro would not accept my password, which I've previously used so often it's ingrained into my fingertips. I thought at first I'd been hacked. It took me a dozen fails to figure out that the computer had somehow defaulted to the Czech keyboard on restarting, and because the Czech keyboard isn't the same as the English keyboard, my password wasn't being correctly entered. I then figured out what keys I needed to press on the Czech keyboard to duplicate my password, and that finally succeeded. I have no idea why the system defaulted to a language other than English. I write in a few languages but I'll be careful to uncheck all keyboards except English when I restart my computer.