Things clarify...

I finally got one clue as to something specific that makes me sound foreign: I, like my brother, have picked up the habit of using "yes" and "no" as bare tag questions, unconsciously, and fairly frequently. Examples from today: "You're working until seven, yes?" and "I get my break soon, no?" Apparently this strikes the typical native English earn as quite foreign. Which makes sense, as I picked it up from non-native English speakers. Of various European ethnicities. So that's one symptom down.