Plural definition is ignored for zero quantity

Plural is Unicode form. all of plural value here. In English, plural for zero like 2, 3,4 so you must if else this value to use others string for this.

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);