Italians : outright joy combined with the immediate switch of language. If from the same region, some family link will be eventually uncovered.
Germans: Slight annoyance. Yet another German; there are so many in every freaking corner of this planet. Maybe a switch to German if nobody else is around and some friendly chatting but not necessarily so.
Belgians – Wallonian and Flemish: Irritation then enjoyable surprise. Speaking English, it is actually possible to communicate with each other and chat about beer, chocolate and the stupidity of Belgian politicians.