I had to translate these 2 questions:
"Is this Chinese (language)?"
"Is this sake?"
And I translated in this way:
それは中国語ですか。
それは酒ですか。
But the book solutions were
それ中国語ですか。
それ酒ですか。
Is this a mistake or it is possible not to use the particle in the polite form? I wonder if it is correct or it's a grammar mistake.
It's the type of “grammatical mistake” that's extremely common in speech. In absolutely proper grammar the “は” should be there, but think of it like “This Chinese?”, people say that all the time but it would be “Is this Chinese?” with more proper grammar.
Also, the Japanese word for “sake” is “[日本酒]{にほんしゅ}”; “[酒]{さけ}” means any alcoholic drink in general and this specific pattern, more commonly used as “お酒” typically means “Is this alcoholic?” in my experience.
As you may know, the omission of particles is extremely common in informal speech. Sentence 1 is the "correct" polite form, while Sentence 3 is a casual form where は is omitted. You would say 1 to your boss, and you would say 3 to your friend.
Sentence 2 is a mixture of both styles, which is also quite common in real-life situations. Politeness is not a black-and-white issue. When speaking respectfully to someone important, you should stick to Sentence 1. However, when conversing with colleagues, Sentence 2 is perfectly natural.
That said, I do think that Sentence 2 can be misleading and inappropriate as an example sentence in a textbook for beginners. Sentence 1 is always the correct choice as a polite form sentence to use in formal situations.
You use the latter form when you indicate something that hasn’t established in the discourse, in other words, that’s just present there but not shared through conversation.
