Without a polarised plug it means current can 'flow' the 'wrong' way through the toaster, backwards through the heating coils.

Which is to say when the toaster is plugged in but off, the heating coils could still be live, and touching them would shock you.

Ah, European standard plugs don't have that. I guess that makes it mandatory to have the power switch interrupt both wires, at least that's what lamps here tend to have.

Polarized plugs have one of the prongs bigger than the other and will only fit one way into an outlet. This ensures that you actually know which wire is hot and which wire is neutral and can ensure the switch controls hot (the power source) rather than neutral (the power return).

Not every country even has polarized plugs, though. The concept was new to me too at a time. The German Schuko plug for example is unpolarized, as is the euro plug, so there's no polarized plug options here.