Most movies have a sense of incomplete. Though usually the story is told, and seen. In about 2 hours. Lyrics that are well written to music and expressed by a suiting voice draws a sense of completion upon those listening. In about 5 minutes. Each time listening, there is a sense of completion. Spare many 5 minutes, for sense of completion.
I think you are trying to say that Sally Yeh is special. She doesn't even understand or speak the Cantonese version of the Chinese language well. And cannot read Chinese language well.
Oh my goodness... Sally's voice made me want to cry 🥲. Chin Wah's singing is a soothing complement - 💧stayed in the eye. Such a lovely duet! Fun Fact: Sally cannot read Chinese. Her native tongue is in Taiwanese Mandarin. Imagine you cannot read in English. But you speak a similar enough dialect, but not even close to English. Then sing a song in English, and make tears from those listening. Sally has that special talent! And Hacken wanted to or was 🥲.
I think you mean they should duet, but not sing at the same time as in a choir (chorus). I think Yeung Tsin Wah passionate singing of this song is an effect or product of having listened to Sally sing this song long ago, for many years. The passion is out of true appreciation and respect for this song, and Sally is the first to give it a meaningful life. Lovely duet!
I like Cantonese, the traditional Cantonese that was made sweeter and more expressive via Hong Kong in later 20th century. It is a more westernized (or more expressive, open) version that did not eliminate the tradition. I think it is more powerful, and touching gently, than the Mainland Mandarin. Sally is Taiwan Mandarin who seemingly fell for Hong Kong Cantonese few decades ago, and had a gentle landing supported by the Hong Kong Cantonese music/lyrics.