When you changed the language setting did you do so specifically for the keyboard?
Go to System Preferences > Keyboard > Input Source and make sure it's British.
Another possibility is that the keys have been remapped, in the same panel you could go to Text, and choose to Replace the apostrophe with a full stop.
Ah I just spotted something! You must be American hence referring to it as a period! You might want to make sure your keyboard is American then!