I can never remember.

The good ones also switch between greyish being off and some color (e.g. green or blue) when on. But not all of them change color, so I tend to toggle them to see if this one of the colored ones -- and thereby confirm what setting it was on.

(ps. In general I do like material and flat designs, it's just these checkboxes I find hard to read)

That might be sensible with a latin language read left-to-right, but for other scripts read right-to-left, it's ambiguous. Is the enabled state then the inverse? (I believe the guidelines are that it should be left as selected for RTL languages)

The problem with trying to do it properly for RTL is that half of systems won't do it properly which is worse than everyone consistently doing it wrong and users learning to cope.

Quite possibly among UI/UX designers, I'd be surprised if most people knew that.

I know I still find it confusing and occasionally have to toggle the switch to both positions before working out which position I want.

Full transparency, I'm neither of those things. I'm saying this out of nothing but my user experience.

Possibly, but why not design widgets so that no such consensus is necessary. Prior GUIs did not need it and now literally billions of people are burdened with it.

Even though UIs were always built on consensus and common experiences users might have from other platforms, I agree that deviating from the norm is a bad idea in most cases. That said, the specific component in question is so common now that for future generations it might become as intuitive as a checkbox.