Pff. Tell the dude to go stuff himself. I do have my own regrets at not knowing much Mandarin and Taiwanese anymore. But the reason for my loss of ability in those languages is because I can speak decent German without spraying saliva all over the place and I will be several hundred times more fluent in English than my cousins who live in Taiwan and think English is cool but can never do more than ape it with a thick accent. In fact, even my mother, when getting snarky comments from her siblings about my sister and my lack of proficiency in Mandarin and Taiwanese, snarked right back how their kids and grandkids were doing in their English classes. She told me about this, sounding rather annoyed, while we were visiting the old home island several years ago.

On top of that, I am pretty passable in Japanese, which lets me communicate with a number of my older relatives and is relevant to my chosen career field. I simply have little opportunity to use Mandarin and Taiwanese anymore--had I not moved to Germany as a little kid, I might have kept up with them, but instead, I just had to learn German and English instead. If any white wannabe Buddhist old man whose Chinese is only marginally better than mine tried to tell me off, I'd just go cuss him out in German and/or Osaka dialect Japanese. I think you'd be quite within your rights to tell him off right back.

Side note: 'guoyi' is a pretty common way of referring to Mandarin, especially by those from the mainland. Native Taiwanese like my family are naturally much less likely to use that term.