Your original passage is highlighted in blue. Everything else are corrections. 

Something about italki.

(Good!)

I am a mechanical engineer.This is my fist time to writ something in italki.

(The first sentence is good. However, most English speakers write "I am" as "I'm". "I am" is formal and used in essays for professors. "I'm" is informal and used when speaking to friends and family. Either one works fine, though! In the second sentence, there are just some minor spelling errors. 
This is my first time writing something on italki.  Your original passage was "to write", but you're in present tense, so it should be "writing".)

I just browsed some items and comments. I found that some comments which just shows a new sentence without explain will confuse language learners. 

(The first sentense is okay. However, it sounds "choppy" to a native English speaker. You want something that flows. So, a better sentence would be, "I was browsing some items and comments." The second sentence is dependent in actuality. You wrote, "I found that some comments which just shows a new sentence without explain will confuse language learners." It's dependent because seperated the first and second sentence with a period.

I just browsed some items and comments. I found that some comments which just shows a new sentence without explain will confuse language learners.

What if we removed the first sentence? Then, it would just be the second sentence. People wouldn't understand where you are coming from. So, I would link the two sentences. Your new sentence would be,

"I was browsing some items and comments, and I found that some comments just shows a new sentence without explaining the correction. This will confuse language learners.

I highlighted, "explaining the correction" because your original sentence was, "without explain will confuse". What are you explaining?)

 

For sometimes the sentences written by language learners is current but just not idiomatic,if no explain,they will not know right or wrong.

(Sometimes, the sentences written by language learners ........ If there is no explanation, they won't know if it's right or wrong. I'm a bit confused on, is current but just not idiomatic, so I skipped it if you don't mind? )

 
So i have some suggestion:
(Good! You sould add a comma before, "I". "So" at the beginning of a sentence is introductory. Your new sentence would be,

"So, I have some suggestions.")

1. Write sentences and give comments using two languages.

(Well done!)

2.No explain , no comment.
(The correct form would be, "No explanation, no comment."

Why we need to learn a foreign language? I thought a lot,The most fundamental must be that we want to know some different,

(The first sentence needs one minor change. Place a "do" between, "why" and "we". The second sentence is a little bit confusing. Is it one sentence, or did you intend to put a . instead of a , between lot and the? 

we want to communicate to each other.we want to know each other.

(We want to communicate with each other, not to each other. For example, I traded with David versus I traded to David. The second sentence is great! Remember to capitalize after a new sentence! :) 

so don't care about if it‘s idiomatic,need care if it is understood.

(So, don't worry about it if it's idiomatic. When someone wants a person to no think of a certain thing, they usually say, "Don't worry about it." Care and worry are relative. However, worry is more fitting in this situation. Some examples,

"Mom, don't worry about it!"

"Jake, you should stop worrying about me, I'll be fine.")

Most of the time, we can understand the mean the writer want to express.

(Almost perfect! Great job on the introductory! Instead of mean, replace it with meaning. Also, it's wants  , not want. When refering in 3rd person and in present tense, the verb comes with an "s". For example, "He wants  a cake." "She bites the pizze."

of course you should point out the mistake in principle if they had.

(Don't forget about the capitalization. It's a minor error, but if you have so many letters that aren't capitalized, your passage, or even paper, will look rushed, and your English professor, if you have one, will be furiated. So, your sentence would be, 

"Of course, you should point out the mistakes in principle, if they had any."

Your correct on, "mistake". Though, majority of the time, there is more than one mistake, so people say mistakes. "David, go over Jake's test and look for mistakes."