volume keys not working

When I change from British English to Polish language (System Preferences/Language & Region) on my MacBook running OS X Yosemite 10.10.3 and perform the obligatory reboot, the volume keys on the keyboard no longer work. I have tried a PRAM reset (Option, Command (⌘), P, and R) to no avail. Internal speakers are selected in System Preferences/Sound but the volume controls are greyed out.

Thanks for any suggestions or remedies.