In my experience of calling the police in Switzerland, and of being present when someone else did so, I'd say the same patterns are here as one can watch on so many youtube videos about calls to emergency centres: the recipient of the call is more likely to be able to dispatch proper help when the information received is brief, clear and to-the-point.

When met with a lot of words, the emergency helper has to find a way to extract the meaning from them, or to get those words to stop, so that the salient points can be gathered systematically.

Having had that peculiar experience of not initially understanding what was said to me, before I realised which language was being spoken, I could imagine that a police (or other) emergency call recipient might, at least at first, and under the pressure of this call plus all else that is going on in the background, say "no" to English, and then after a few minutes his/her English kick in, so that the conversation becomes possible.