The other day, I came across a young couple who I didn't know which country they were fromappear to be from my region/country, but I felt excited because it's rare to meet people from other countries in my city. The other day, I came across a young couple who didn't appear to be from my region/country, but I felt excited because it's rare to meet people from other countries in my city.

The way you had it written initially, though understandable, is a little awkward to read/say. I modified it to sound more natural.

Other people and I shouted at the same time in our dialect, "sSee, foreigners!" Other people and I shouted at the same time in our dialect, "See, foreigners!"

Unexpectedly, the two people said in Mandarin, "nNo, we are not foreigners, we are Chinese". Unexpectedly, the two people said in Mandarin, "No, we are not foreigners, we are Chinese".

I caouldn't believe that they can speak Mandarin and can understand our dialect. I couldn't believe that they can speak Mandarin and can understand our dialect.

The story is in the past, so the sentence needs to be in the past tense.

I didn't know what to say or how to say it. I didn't know what to say or how to say it.