The main reason I loved learning English is that it's very useful for learning almost any kind of things.

While using the past tense "loved" here isn't necessarily incorrect, it does make it sound as if English is something that you are no longer studying. Given that you posted this journal on this site in English, I would say you very much still are learning English!

The lLearning resources for people who don't understand English would bare very limited as in most fields the best learning resources would bare in English and reading in the original languageversion is better than reading in the translated version.

While I did not break it up, this is a very long sentence and would benefit from being broken into separate sentences.

I used to love reading mangas so I started learneding Japanese.

Again, you are continuing to improve your Japanese, so learning should be present tense, not past.

I don't read mangas so as much now, but asince I live and work in Japan I have to improve my Japanese constantly.

"As" is not incorrect but sounds somewhat overly formal.

But after a month I quit.

This is not incorrect for casual speech, but starting a sentence with a conjunction is technically grammatically incorrect and should not be done in formal contexts.

In November of last year, I re-started to learn iting it again because I became a fan of a Korean musician.

I haven't learnstudied Cantonese forin several years as I think there is no need to spend time learning it anymore because I can understand most of them(the it (slangs excluded as there areis too manyuch difficult slangs) and I don't have any friends who speak Cantonese.

Again, this is a very long sentence that may benefit from being broken into multiple sentences.

I learned it by the influences ofthrough songs, TV dramas, and the Yue Opera of Hong-k Kong.

I'm not quite sure what you mean here. If you mean that you learned it using songs and the like, the above correction is most appropriate. If you mean you learned it because of songs and the like, this would be more correct but somewhat unclear: "I was influenced to learn it by songs, TV dramas, and the Yue Opera of Hong Kong."

In the latter, it's unclear what is modifying what, so it would be best to rewrite it entirely. For example: "Listening to songs, watching TV dramas, and seeing the Yue Opera of Hong Kong influenced me to learn Cantonese."

I have the experience ofd learning English, Japanese, Korean, and Cantonese.

The main reason I loved learning English is that it's very useful for learning almost any kind of things.

The learning resources for people who don't understand English would bare very limited, as in most fields the best learning resources would bare in English and reading in the original language is better than reading in the translated version.

I used to love reading mangas, so I learned Japanese.

I don't read mangas aso much now, but asince I live and work in Japan I have to improve my Japanese constantly.

I started learning Korean 4 years ago, because I was curious about Korean characters.

But after a month I quit.

this should probably be included with the last sentence, like this:
"I wanted to know how to read them, but after a month I quit."

In November of last year, I re-started to learn it because I became a fan of a Korean musician.

It would probably sound a little more natural to say "I started to learn it again" instead of "I restarted to learn it."

I haven't learned Cantonese for several years as. I think there is no need to spend time learning it anymore because I can understand most of them(the slangs excluded it (except for slang words as there are toso many difficult slangones), and I don't have any friends who speak Cantonese.

There are many ways to break this sentence up. I have only shown one possibility, but I think it definitely needs to be separated into a couple of sentences.

I learned it by the influences offrom songs, TV dramas, and Yue Opera of Hong-kKong.