2. I agree, there's no big difference, but the first way sounds better because it's more concise. 3. The forms are correct, but 'mustn't and 'shan't' are obsolete (not used often) modal verbs now.

1. Both are fine and correct. 2. There is no big difference but I would probably go with the first one in this context - it sounds better with "Welcome to". 3. Shan't 4. It uses "language" because "body language" is an abstract noun - it's uncountable so there's no need to make it plural. If you were using "body language" to refer to an individual or community's set of body movements, which isn't wrong, then it would be fine to make it plural. 5. No, we use the plural for countable nouns and the singular for uncountable nouns. E.g.: "I don't have any dogs." or "I don't have any money."

1."It's not at all polite!" VS "it's not polite at all!" - both are correct, although here, the second one is more common 2."Welcome to the New York City Lost and Found Office." VS "Welcome to the Lost and Found Office in New York City."(both are right? Is there any difference between them?) - the first one, as it is the name of the department. We don't use "in" unless we are comparing it with somewhere else, as "The Lost and Found Office in New York is better than the one in San Francisco" 3.is-isn't, do-don't, did-didn't, must-mustn't, shall-? - "shall not" - there is no contraction involving shall 4."People from foreign countries have different body language from us." Why not use "body languages"? - there is only one "body language" 5.When "any" in the negative sentences/interrogation questions, we should use the noun like this,dog,pen,book,but not dogs,pens,books,yes? Like this...We don't have any pen.(right) We don't have any pens.(wrong) - this is not correct, with "any" you need to use plurals, "We don't have any pens" is right 希望这个可以帮助