uncountable nouns 後面真係可以加s e.g. I want two cokes, please.

May I have 2 cokes, please? 完全對.

指明的uncountable noun 要加s
例:
Indonesian, Malaysian & Korean are my favorite foods.
Indonesian, Malaysian & Korean are nice peoples.

什么情况会在 uncountable nouns 后面加S呢? 就是当这个名词是解释不同种类的时候.

比如:
1) There are many fish in the sea. 海里有很多鱼.
2) There are many kinds of fishes in the sea. 海里有很多不同种类的鱼.

May I have 2 cokes, please? 这句可以对, 也可以错, 要看你要表达的意思. 如果那里的确有两种可乐, 而你又真系要两种, 就可以系对的(比如那里有百事可乐也有可口可乐). 但是如果只有一种可乐, 就是错了.

Correction :
May I have 2 cans of coke, please? or May I have 2 bottles of coke, please?

通常uncountable nouns後面不會加s，如water, ice , oil, tea, glass等，如glass若加-es就變成眼鏡之意，water加-s-就用作江河等意。
而你所說May I have 2 cokes, please? 應該為　May I have 2 cans of coke, please?

you can put the s .
Beause this word is a uncountable nouns and the Coke are 雙數