He grew up playing in the narrow, crowded streets of Manhattan’s Chinatown. He has lived and worked there for all his 61 years. But as Wee Wong walks the neighborhood these days, he cannot understand half the Chinese conversations he hears.

Cantonese, a dialect from southern China that has dominated the Chinatowns of North America for decades, is being rapidly swept aside by Mandarin, the national language of China and the lingua franca of most of the latest Chinese immigrants.

The change can be heard in the neighborhood’s lively restaurants and solemn church services, in parks, street markets and language schools. It has been accelerated by Chinese-American parents, including many who speak Cantonese at home, as they press their children to learn Mandarin for the advantages it could bring as China’s influence grows in the world.

But the eclipse of Cantonese  in New York, China and around the world  has become a challenge for older people who speak only that dialect and face increasing isolation unless they learn Mandarin or English. Though Cantonese and Mandarin share nearly all the same written characters, the pronunciations are vastly different; when spoken, Mandarin may be incomprehensible to a Cantonese speaker, and vice versa.