There is no doubt that English is important for me, as a Chinese person.

Only certainly nationalities (mainly those ending in -en or -an sounds) work as singular nouns on their own. Other nationalities (including Chinese) are adjectives only.

I could learn more about the world by using English.

"by" expects a verb after it in this usage.

I would have a talk with (the Americans or British / Americans or British people).

Ok, so this is pretty complicated, but here's a rough explanation for why the two options presented are more natural sounding.

Some nationalities are nouns and adjectives (mostly ones that end in -en or -an sounds - e.g. American)
Some nationalities are adjectives (e.g. British, Chinese)

To make the noun nationalities a collective noun, you just have to pluralise them like "Americans"
To make the adjective nationalities a collective noun, you either need to use them as an adjective with a noun like people (so "British people" or "Chinese people") or prefix them with "the" like "the British" or "the Chinese".

You can also use this "the" with the noun nationalities too like "the Americans", but you don't have to.

However, using "the" with a nationality puts a bit more distance between them, like it makes people envision a group divided by nationality. This could mean that you're implying the person you're talking to is a representative of their country in some way, or of e.g. your company's American branch office. However, in the wrong context making them a collective noun with "the" can also sound kind of rude or insulting, like you're making them separate when you don't have to. So it's safer to use them as adjectives and go with "British people" etc.

Finally, if you're going to combine them in a phrase, it's better to treat them the say. So "the Americans or British" works because the "the" is being applied to both "Americans and British" so they're both turned into collective nouns by the "the", but if you use "Americans or British _people_" it's better to leave off the "the" as "British people" and "Americans" are sort of the same category of collective noun.

I would read some dissertations forto learning about this world better.

But how to write well is the biggest challenge for me I've ever met.

"I've ever met" already implies "for me", so you can omit the "for me".

I found it really difficult to write a chapter without mistakes.

A chapter is still a lot of writing! Your typical English speaker probably has to correct a couple of mistakes if they write a whole chapter.

I would (get better / improve in) at English if my writting was getting better.

"get better at" is a little more casual than "improve at" but either works. You do need to pick one however.

Theoday's weather of today was not good and the temperature (was getting downdecreasing quickly / decreased suddenly).

"suddenly" has the connotation of something happening in one moment, so it's unusual to applying to verbs in progressive (-ing) tense.

So I haved to put on a coat before I left home.

"left home" is past tense, so "had" should be too.

I hope that the weather is getting better so that I couldan enjoy outsides camping.

"so that I could" is explaining the reason you did something in the past

"so that I can" is for ongoing or future actions

There is no doubt that English is important for me, a Chinese person.

I removed an extra space before the word, important. Remember to include a space after a comma. Chinese is an adjective, so I added a noun, person.

I could learn more about the world bythrough English.

I would have a talk with the American or British people.

I would read some dissertations forto learning this world better.

But how to writeWriting well is the biggest challenge for me I've ever met.

I fouind it really difficult to write a chapter without mistakes.

I would get improve in English if my writting was getting better.

Theoday's weather of today was not good and the temperature was getting downdecreasing suddenly.

So I haved to put on a coat before I left home.

I hope that the weather is gettinggets better so that I couldan enjoy outsides campingcamping outside.

There is no doubt that English is important for me, a Chinese person.

I couldan learn more about the world bywith English.

I would have abe able to talk with the American or British people.

Or, alternatively: "I would have a conversation with American or British people"

I would get improve inmy English if my writting was getting better.

The weather of today was not good, and the temperature was getting downdropping suddenly.

So I haved to put on a coat before I left home.

have to: present tense
had to: past tense

I hope that the weather is gettingwill get better so that I couldan enjoy outsides camping.