外國人既英文俗語

Actually, it doesn't matter.
Modern English is not like Chinese, there is no clear rules distingushing between spoken and written language.
Making use of your example, native speakers would say "yup", "ya" or things simular to express an affirmative meaning rather than using "yes". But it is actually normal and common to say "yes" simply, it doesn't make you odd or weird.
But of course, if you can learn more about common spoken English including the phrases and vocubs, communication will definitely be easier and a easeful circumstance can be created more easily.

您好，我推薦您一個非常不錯的英語學習網站，里面有很多非常不錯的學習資源，您能從中得到不少幫助：

http://www.hkenglishstudy.info

祝您好運!

其實外國人講yes 會講左"yea" 或者 "yup" 多數會係口語..但係"yes"就會書面同口語都得...