sīn náaih ga fē
The Cantonese for latte is 鮮奶咖啡, fresh milk coffee (鮮 is fresh, 奶 is milk, and 咖啡 is the loanword for coffee.
The Mandarin word for latte is simply the loanword 拿鐵 (the Mandarin pronunciation ná tiě sounds more or less like latte (the Cantonese pronunciation nàh tit no so much), or the fuller 拿鐵咖啡 (ná tiě kā fēi), latte coffee.
Personal Note
During a trip to Macau, one of my colleagues unsuccessfully ordered a latte. This entry (and this app) would have been handy!