On two occasions, I was the one to be surprised.
The first one was over a year ago when, after I started singing the first line to the birthday girl (an English girl in her 20s), she then joined in with the Cantonese version. We finished the song together, me in Mandarin, her in Cantonese. She'd attended school in Hong Kong in her teens!
The second one was last week when I was helping out at the floating pub Tamesis Dock, moored on the south shore of the Thames. On the upper deck was a big group of mainly Spanish speakers, celebrating someone's birthday. When I finished singing the song to the birthday girl (a Spanish speaker), she turned round and said, "谢谢你,阿姨 / xièxie nǐ, Āh Yí / Thank you, Auntie!" I was so taken aback: "你会说中文!你怎么会说中文?!? / You can speak Chinese? How is it you can speak Chinese?" She said, "当然啦,我在北京住过三年! / Of course, I'd lived in Beijing for three years!"
(London, 2018 / 2019)