According to the documentation :
The selection of which string to use is made solely based on grammatical necessity. In English, a string for zero will be ignored even if the quantity is 0, because 0 isn't grammatically different from 2, or any other number except 1 ("zero books", "one book", "two books", and so on).
If you still want to use a custom string for zero, you can load a different string when the quantity is zero :
if (commentsCount == 0)
str = res.getString(R.string.number_of_comments_zero);
else
str = res.getQuantityString(R.plurals.number_of_comments, commentsCount, commentsCount);