Should all Japanese directly address foreigners in Japanese ?

[Maciamo]

Ands I suppose it's a stereotype that foreigner = english speaking person.

Yes, that's also why I thought that it didn't make much difference whether the Japanese asked "nihongo wo wakarimasu ka" or "do you speak English?". Over half of the Westerners do not speak English as their mother-tongue, and maybe 1/3 of this half cannot speak English at all.