What percentage of the San Francisco Chinese community speaks Mandarin as opposed to Cantonese?

My understanding has always been that Chinatown in SF is generally Mandarin and the Chinatown in Oakland is generally Cantonese. I recently found out that there is a large Cantonese population in SF, but not in Chinatown.

Then again, I don’t have terribly much to back this up, and the language spoken in SF is always too fast for me to pick out the features I know are one or another.

The cool thing with the Chinese language is that the written language is unified throughout all the dialects, but the way those characters are pronounced varies regionally. Some of it is minor differences (similar to the Boston or Southern accents in the US), but the major regional shifts (Mandarin and Cantonese being the two most commonly spoken in the US) are enough that speakers cannnot understand each other.

Though written chinese is a single language as bpeoples correctly says, there are two forms of characters: Traditional and Simplified. Cantonese speakers (such as the bulk of people in Hong Kong) use the Traditional, while Mandarin is more frequently in Simplified form.

I am from San Francisco, so I got curious about your question and decided to do some research. It wasn’t easy finding statistics on this level.. the government doesn’t differentiate Chinese languages on the census. This article is a little dated (2003) but it can give you a general idea, I think.
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Chinese communities shifting to Mandarin

A gradual shift from Cantonese, a dialect spoken in southern China, to China’s official language of Mandarin, has been taking place in America’s Chinese communities. These days, Mandarin’s growing influence can be heard even in San Francisco’s Chinatown, long a bastion of Cantonese speakers.

“Now, nobody pays attention because it’s so common,” said Pak, a longtime Chinatown activist and consultant for the Chinese Chamber of Commerce who speaks both languages. Though Cantonese remains Chinatown’s primary tongue, many shopkeepers speak at least a few words of Mandarin.

Statistics document the shifting landscape: A 1986 consumer survey found almost 70 percent of Chinese households in the San Francisco area spoke Cantonese; 19 percent spoke Mandarin. A survey last year showed the divide narrowing to 53 percent Cantonese and 47 percent Mandarin, according to a study for KTSF, a television station that devotes most of its programming to Asian-language shows.