Which currency am I charged in?

The currency shown and charged follows your location, not the site's language: visitors in the US see and pay in dollars, everyone else in euros.

The currency toggle lets you switch the display to the other currency manually. Checkout then shows and charges exactly the amount that was displayed beforehand — the price shown is the final price, there is no later conversion.

An extension is always billed in the same currency the event was originally purchased in.