There are also so many more layers to this, especially in how Mandarin vs Cantonese vs English is used
So as a preface: Cantonese and Mandarin are two dialects of Chinese but they are fairly different grammatically, and while many people in Hong Kong and Guangdong (Canton) region speak both, understanding one dialect if you only speak the other is more or less impossible. Native speakers of one often treat the other as a second language.
so in the prime universe, Evelyn speaks Cantonese and only Cantonese with prime!Gong Gong, who is never seen speaking any other languages aside from Cantonese. He barely understands Joy's broken Mandarin and only offers a berating response to her in Canto. Waymond and Evelyn's Chinglish is a mix of Mandarin and English together, but they mainly speak English with Joy, and she can barely understand her mother tongue.
between prime!Gong Gong and prime!Waymond, Waymond is only ever talked at. Gong Gong speaks to him in Cantonese (which he presumably understands) but he only replies back in Mandarin once. There is very little direct communication between them without Evelyn.
so you have a major language barrier between Joy and her grandfather and minor language barrier between Waymond and Gong Gong - they have the means to communicate and could understand eachother, but it takes tremendous effort to do so.
and Evelyn has to navigate all three worlds, between her daughter, her father, and her husband, and be the bridge between them, as the balance and the tether and their touchstone. Which is such a brilliant metaphor for generational disconnect here and how Evelyn really is in the centre of all that chaos —
but it doesn’t stop there.
most crucially, Mandarin and Cantonese are used for the emotional and private dialogues between Evelyn, Waymond and Gong Gong. In all their flashbacks, in the kung fu/movie star universe, and also when Evelyn and Waymond talk about their domestic life in the IRS building - their divorce, their marriage, their daughter - conversations about the matters of the family are in Chinese.
Which leads us to: the Alphaverse!Wangs all speak English with each other and exclusively in English. Alphaverse!Waymond calls Alphaverse!Gong Gong “sir”, Alphaverse!Gong Gong tells prime!Evelyn to kill Joy in English. The only time Evelyn talks to alpha!Gong Gong in Cantonese is when she confronts him about letting her go.
English is depersonalised in the movie — it’s used practically and for exposition and to communicate quickly, while Chinese usage is infinitely personal. (Really, the first clue that we get that Waymond is...different is when he speaks to Evelyn in English with no Chinese at all).
And the most ironic thing about it all: the universe where the Wangs all understand and communicate perfectly is also where the family is most broken - Alphaverse!Waymond thinks that his own daughter is no longer capable of mercy or worthy of saving. Alphaverse!Gong Gong thinks he has to kill Joy in every universe.
Yes, the remaining Alphaverse!Wang family all understand each other, but without Evelyn, without their mother tongue, they might as well not be a family at all.