I learnt traditional chinese too but I think people here to learn chinese have enough to do with the simplified logograms ;-)
My daughter has chinese course in her school. Of course, I would like her to know the traditional writing but I m happy if she only knows the simplified one because she's been making great effort to memorize and mostly she writes with pinyin.

Thanks for the traditional !

I think this kind of thing is a constant difficulty in translation: is it supposed to be the meaning as it would be expressed in the translated language that is being given, or is the translation a hybrid of meaning plus a somewhat literally translated string of words?

Chinese is notoriously short of surnames, so there is a tendency, I believe, to refer to people by their profession or some other characteristic.

how do you know it is a sir

It's true, and I think that too. Buuuuut, I've seen this a lot is comment sections saying that duo is just using their country's language. But just hover your mouse or tap your finger on a word that you're not sure, and it will tell you the English translation of it. If you're just not sure. I usually hover my mouse over words like teacher, doctor, men, women and so on. So if you're ever having trouble, just tap of hover over the word! :D

this should be accepted.

This course is taught in pinyin, so it should be Li. Lee is a common spelling in Korean and Cantonese. Other countries, such as Singapore, Hong Kong and Taiwan dont use pinyin (hence Bruce Lee (李), whose parents were from Hong Kong and spoke Cantonese)

You would not say it that way in English

特别如果李老师是你的老师，可是我们不知道是谁问这个问题。可能李老师的老板问，或者李老师的同志，或者一位警察。

Especially if Li is your teacher, but we do not know who is asking the question; maybe, Li's boss (the Principal) is asking, or Li's coworker (perhaps another teacher senior to Li), or a police officer. I think it's fine to use "nin" rather than "ni" here, but "ni" is acceptable, depending on the context.