With a landline,the answer to that is obvious.So how does it know which country you are in?
However, I see your point about a mobile in the UK which connected to a tower in France. I believe that the behaviour plugwash described is 'reciprocal', so I think that a call to '999', as well as 112 (and maybe 911 etc.) received via a tower which implied that the phone was in France would get connected to the French emergency services (not a recorded message in French), and I would imagine it is probable that the operator who answered (and/or one of their colleagues) would have a reasonable command of English!

