Quote:- "So, are they Malaysian or Chinese? And if they are Malaysian, why are they singing a Chinese song in Chinese?"
The girls are, in terms of nationality, Malaysians.
"...why are they singing a Chinese song in Chinese?"
To understand why this is so, you need to know a bit of history, otherwise you'll be quite confused as you are now.
Malaysia, (i.e. Peninsular Malaysia) was a former British colony which became an independent nation in 1957, and joined later by Singapore, Sabah & Sarawak, (i.e. British North Borneo), in the early-1960s to form Federation of Malaysia (Singapore left the Federation and became an independent city state in the mid-60s)
When Malaysia was a British colony, the British administrators imported, (the technical term was "indentured"), thousands of workers for their rubber plantations, tin mines, factories from China and colonial India. My Hakka grandfather, from Guangdong Province, was among them.
These early 18th, 19th Century wretchedly poor illiterate Chinese and Indian workers had to build Chinese and Indian vernacular schools and imported Chinese and Indian teachers from China and India to teach their children in order not to lose their "Chineseness and Indianness" A small number of these Chinese vernacular schools, (which gives Chinese medium education for the first 6 years), still exist today alongside the "National Type" schools where the medium of instruction is the Malay language.
So, the ethnic Chinese girls in the video obviously attended these vernacular schools for 6 years and "switched over" to the National Type schools for their "High School" education to learn more Malay and some English.
Here's a biodata of one of the girls.
Because of the country's colonial history, and the mass importation of Chinese workers principally from Guangdong and Fujian Provinces, the ethnic chinese population, numbering at present about 10% of the total population, (in a total population of about 32 odd million), could speak at least one Chinese dialect, mandarin, Malay and English. I myself in my old household had to speak Cantonese with my mother, Hakka with my grandfather and my English educated father in English.
The video is obviously shot on location somewhere in China and judging from the "deserted" streets and buildings in the background, it was obviously a purpose-built movie set to film period movies.
The "中国上海东海影视乐团" was merely the name of the orchestra. (it wouldn't make economic sense to bring a whole orchestra from Malaysia to China to shoot a music video.