I am no native english speaker, so it probably does not really apply to me, but the language I speak to people is mostly force of habit.

There are people I started speaking german to from day one, and with whom I only speak german, unless someone who can't understand german is with us.
There are others who, despite being german mothertongue, spoke english to me from day one, therefore I never bothered speaking german to them.

I think the habit is critical towards understanding: I understand extremely well those with whom I always spoke german with, despite they come from the four corners of the german speaking area (swissgermans, austrians, south and northern germans) but just because my brain has adjusted to their accent. And all this in proper german only, obviously, as swissgerman is not for me. No no no.