Hello, interesting issue, really.
I think we should have in the flight strips some kind of remark, such as "english-speaking" or something like that.
When a spanish company carrier calls in in english, i usually mark it in the flight strip, so i (or my next colleague) can remember later, and when i hand it off to the next frequency i usually tell my collegue: "the spanair in english, with you".
But we should have in the flight plan, i agree.
Best regards.