The word "says" is correct and usable in written English. It can be used as a verb, usually used in the third person singular form. For example: "Sarah says she would like to visit Mexico this summer.".
"Yeah," he says.
"Yes," he says.
Who says?
"Of course", he says.
"I don't know," he says.
"Yes," she says.
"Definitely," he says.
Being a terminologist, I care about word choice. Ludwig simply helps me pick the best words for any translation. Five stars!
Maria Pia Montoro
Terminologist and Q/A Analyst @ Translation Centre for the Bodies of the European Union