Czechs tend to learn UK English in school but thanks to the likes of YouTube, Netflix, et. al., American culture and slang have become increasingly widespread. Depending on the Czech company or individual client you may be asked to teach a specific dialect, especially if a student is preparing for an exam that relies on mastering one or the other. Teachers in this situation may find themselves relearning not just vocabulary (sweater vs. jumper, for instance) but collective nouns and auxiliary verbs right along with the rest of the class.