Key Phrases Recap:

“flashy” = 派手
“essential” = 必要な
“humid” = adj.
“humidity” = n.

Yuko: “As you know, summer of Japan is always humid.”

This should be:

“As you know, Japan's summers are always humid.”

Proper names, such as the names of people or countries, take the possessive case ['s]. Otherwise, it sounds strange. For example: “book of Jack, house of Ruriko, etc.” sounds very strange in English because Jack and Ruriko are proper names. This should be: “Jack's book, Ruriko's house, etc.”

‘summer’ is a countable noun. Because you are talking about all summers in Japan, the plural form of the noun is more appropriate here. The verb is changed from ‘is’ to ‘are’ to match the new subject.

Yuko: “Just I open the windows.”

This should be:

“I just have the windows open.”

‘just’ is an adverb and almost always occurs immediately after the first helping verb. If there are no helping verbs, then it occurs directly before the main verb.

“to have the windows open” means you previously opened the windows and they are still open right now.

Jared: “Why don't you like air conditioners?”
Yuko: “Because I feel tired.”

This should be:

“Because they make me feel tired.”

“I feel tired” means you feel tired right now! The actual reason you don't like air conditioners is because the air conditioners force you to feel tired.

Yuko: “If I ride crowded train that is terrible, so I think air conditioner is essential.”

This should be:

“If I ride a crowded train the humidity is terrible, so I think air conditioners are essential.”

‘train’ is a countable noun and requires an article. Because you are talking about trains in general and not a specific train, ‘a’ is the most appropriate article.

‘that’ can only be used to represent a topic that both the speaker and listener have agreed is the main topic already. Since ‘the humidity’ was not the topic of our conversation, you can't use ‘that’ here.

‘air conditioner’ is a countable noun. Since you are talking about all air conditioners in general, using the plural of the noun is appropriate. Therefore, the verb changes from ‘is’ to ‘are’ as well.

Yuko: “But really in Japan people say air conditioner is bad for human body.”

This should be:

“But really in Japan, people say air conditioners are bad for your health.”

As in the last sentence, ‘air conditioner’ is a countable noun. Since you are talking about all air conditioners in general, the plural form of the noun is more appropriate here.

We don't really say something is good for the ‘body’ unless it is directly good or bad. For example:

“Too much sun is bad for the body.” (the body gets burnt)
“Drinking milk is good for the body.” (the body grows stronger)

We use the world ‘health’ in more general cases. “good for your health/bad for your health” are fixed expressions.

Yuko: “It is a pretty famous company at 3D CG world.”

This should be:

“It is a pretty famous company in the 3D CG world.”

In terms of “worlds,” we think of each world as a sphere. People exist within the sphere. Therefore, ‘in’ is more appropriate than ‘at’ for this sentence.

‘world’ is a countable noun and requires an article. Because you are talking about a specific world - 3D CG - the article ‘the’ is the most appropriate one to use.