I mostly speak English with my friends in Japan, but Japanese finds its way into nearly every conversation. Mostly from them, but sometimes from me. Just simple everyday things.
But one of these things managed to find its way into nearly every conversation with my English-speaking friends: え〜 (eh, expression of surprise). It’s great for any time when someone says something unexpected. Also, quite versatile. Prolonged え〜 is more like a 知らない (shira nai, I didn’t know that) vibe, whereas a very quick えっ is more like whoa!
But there’s also another version to look out for, へ〜 (he-eh) which sometimes indicates feigned surprise.