I am learning German with a teacher in Braunschweig. Although she is native and very experienced in German teaching, sometimes her suggestion was not the same as those in our textbook, which was just published in these two years. For example, she told us that the plural form of "der Cappuccino" is "die Cappuccinos", but the textbook suggested to say "die Cappuccini". Does it mean that one of them was wrong? If an usage can be wrong, is there a standard or official version of German grammar?
This is still a democratic society, and there is no such a thing as an "official" German grammar. However, the Duden dictionary has a status of being the accepted reference book for spelling. Duden however is not so much normative, rather descriptive: a word or spelling that is around long enough will eventually make it into the Duden.
In Duden. Deutsches Universalwörterbuch A-Z, (1816 pages, ed. 1989) you find:
Cappuccino, der; [-s], -[s] [ital. cappucino, zu: cappuccio = Kapuze, nach der Farbe der Kutte, die von Kapuzinermönchen getragen wird].
You may also use Cappuccini, but you would probably do this only when you want to display your knowledge of Italian. Or you are in an Italian restaurant and say:
Due cappuccini, prego!
But attention! When you are in a restaurant or pub and you want to order multiple of them (using German, by way of variation) you may simply say
Zwei [drei, vier... hundert] Cappuccino bitte!
without making a mistake. This is like
Zwei Bier bitte!
as opposed to
*Zwei Biere bitte!
which would be the "logical" form, but nobody says this.