Chinese- Mandarin -i dunno?

Happy birthday big brother

Chinese = Text
Mandarin= the language spoken
"san Yat Fie Lock A GOH" = cantonese "happy birthday Big Brother

SAN = Birth
YAT = Date
FIE -LOCK = Happy
A GOH = Dude/Older brother/a friend

It means what they say and it is Cantonese.

Chinese is a broad concept to refer the language spoken in China, two of the most important are Cantonese (Hong Kong, Guanzhou, etc) and Mandarin (the rest of China).

Mandarin as here they say, it is a dialect, but often refered in Western as the "Standard" Mandarin that is the official language in China (the Mandarin dialect spoken in Beijing).

Many people outside China always think China language is Cantonese (just watch all foreigner movies when they play any Chinese part, the Chinese you hear is actually the Cantonese who is spoken only in few parts of China)

"San Yat Fie-Lock A-Goh":
1) Happy Birthday, dude
2) Happy Birthday, big brother
...depends on the context.

And by the way, Chinese is the Language, Mandarin is the dialect.

don't worry, you're friend's saying "happy birthday big bro!"

'happy bday d'bag'