It is more accurate to say that we use the BE going+ to future for a future situation for which we have present evidence. There can be no question of intent in an utterance such as, "Look at those black clouds; it's going to rain". Note than when the present evidence is simply the knowledge in our own minds, then there is no effective difference between the present continuous and the Be going+to structure.
"I am learning Chinese" is possible here.e.g Before I go to China next year, I'm going to learn some Cantonese.(rather than ....I am learning some Cantonese).
Question 1. As native English speakers, will you think it's too complicated to differentiate "an INTENTION to do something in the future with no definite arrangement' and "with a DEFINITE ARRANGEMENT"?.
We don't need to.
Question 2. When you communicate in English, will you consider the above factors before using "going to" or "the present continuous"?
We don't do it consciously, but there is some process in the brain that leads us to one form rather than the other.
Quesion 3. Do you think English is difficult for non-native English speakers to learn because there are a lot of rules of grammar have to be followed?English almost certainly has no more (and no fewer) 'rules' than most other languages.