> You said the tests should be in the implementing languages

No, I did not say "should", I said that they would "likely" be in the same language, for reasons given above. Yes, I get that there are also reasons to deliberately choose otherwise.

> but when you have to test functionality across services, you might well not want to pick a technology that the service(s) under test were built in.

Great. For the third time, what is the case for Newman being that technology, over a well-known programming language?