I will give you ways to make your writing more simple by writing less (fewer adverbs, shorter complete sentences). Also, when you speak shorter sentences it is easier to do.

One month has passed since I arrived in the  UK

It's been a month since I arrived from Japan.
Studying children's literature is much harder than I expected.
Much more reading, and much more thinking deeply. I need to read more and think more deeply.
But people are nice and I love the nature surrounding me.
I think I can say that  I've started to settle down. and I have become used to living here.

However, I find it really difficult to communicate with other people deeply.
I can understand what my friends are saying, but  I can't say much myself.
I wonder how bilingual people think when they talk. 
Do they think in both languages at the same time?
For me, When I talk to people, I have no time to think in Japanese and translate it to English, so I feel like I am talking without thinking.
I practice by talking to myself in English, but I usually lose track of what I was trying to say.

I have not discovered the most efficient way to become fluent in English, so I need to practice speaking English daily. 
In Japanese, they say "習うより慣れろ, " meaning you learn best by doing.
So I'll try to make opportunities to talk with people in English.

It's difficult for me to keep a conversation going if there is I have nothing in common with the people whom I talk to.
At the moment, I have more international friends than local friends, since we have the same goal: to be a good & clear English speakers. and We also we are facing  face the same challenge: living and studying in the  a foreign country.
I don't think that I can improve my communication skill dramatically, but I'll work steadily.
Starting  I will start by doing my homework with my full effort!

You did a great job sharing your thoughts.

It is not possible to listen to a conversation in English, come up with something to say in Japanese, translate that to English and speak that English translation in the conversation. This is especially difficult to do with Japanese and English because the word order is so different between the two languages. The most efficient/effective way to converse in English is to think in English as you listen, think in English as you form what you want to say, and then say it in English very soon after the speaker stops speaking. Cognitive linguists believe that people who speak different languages actually think differently because they organize and process their thoughts differently according to the particular language they speak. For you to speak English fluently, knowing vocabulary and grammar isn't enough. You must organize and process your thoughts in a way that matches the English language. You must practice thinking whole thoughts (phrases, short sentences) in English.

One month has passed since I arrived in the UK

It's been a month since I came here from Japan.
Studying children's literature is much harder than I expected.
Much more reading, and much more thinking deeply.
But people are nice and I love the nature surrounding me.
I think I can say that I've started to settle down and become used to living here.

However, I find it really difficult to communicate with other people deeply.
I can understand what my friends are saying, but I hardly have anything to say back.
I wonder how bilinguals think when they talk.
Do they think in both languages at the same time?
For me, when I talk to people, since there is no time to think in Japanese and translate it to English, I feel like I talk without thinking.
I practice by talking to myself in English, but I usually lose what I was trying to say.
Maybe there is no way to understand the most efficient way to communicate in the second language, so I guess I need to practice communication on a daily basis.
In Japanese, they say "習うより慣れろ, " meaning you learn best by doing.
So I'll try to make opportunities to talk with people in English.

Then again, it's difficult to keep a conversation going if we have nothing in common.
At the moment, I have more international friends than local ones, since we have the same goal: to be a good & clear English speaker, and also we face the same challenge: living and studying in a foreign country.
I don't think that I can improve my communication dramatically, but I'll work steadily.
Starting by doing my homework with full effort!

Very well written!

One month has passed since I arrived in the UK. 

It's been a month since I came here from Japan.
Studying children's literature is much harder than I expected.
Much more reading, and much more thinking deeply.
But people are nice and I love the nature surrounding me.
I think I can say that I've started to settle down and become used to living here.

However, I find it really difficult to communicate with other people deeply.
I can understand what my friends are saying, but hardly have anything to say back.
I wonder how bilingual people think when they talk.
Do they think in both languages at the same time?
For me, when I talk to people, since there is no time to think in Japanese and translate it into English I feel like I'm talking without thinking.
I practice by talking to myself in English, but I usually lose track of what I was trying to say.
Maybe there is no way to understand the most efficient way to communicate in the second language, so I guess I need to practice communication on a daily basis.
In Japanese, they say "習うより慣れろ, " meaning you learn best by doing, so I'll try to make opportunities to talk with people in English.

Then again, it's difficult to keep a conversation going if there is nothing common with the people that I talk to.
At the moment, I have more international friends than local friends, since we have the same goal: to be a good & clear English speaker; and also we are facing the same challenge: living and studying in the foreign country.
I don't think that I can improve my communication skills dramatically, but I'll keep working steadily.
I'll start by doing my homework with my full effort!

What a lovely post!. Keep up your hard work - it must be challenging to be in a different country,  but you can do it!