Not just change how they speak, but learn entirely new concepts that do not exist in their language, and that they have no intuition for. I'm currently teaching German to somebody whose native language has no tenses, no cases, mostly no plurals, essentially no genders, different phonemes, and is written in a different script.

Having to not just learn these concepts, but be familiar enough with them to form an intuitive understanding and use them correctly in most instances, at a near-native level, is something that takes a decade or more for an adult. To be dismissive of somebody who makes that effort is pathetic.