> Is there a reason why that text needs to be sent before the user clicks the "translate" button?
Yes - UX latency. I would expect this kind of thing to take a few thousand milliseconds, and shaving off a few hundred milliseconds from between when the user highlights text and when they select "translate" is significant. The fact that this data is being sent to Wikipedia of all places further signals that the usage is likely to be innocuous.
Do I think that this is globally a good design decision? No, for both engineering and privacy reasons. There's definitely no good reason why it should be sent to Amazon at all.
