In Japan, it is really hot and the average temperature in Japan in August is 26.4℃. In Japan, it is really hot and the average temperature in August is 26.4℃.

Don't need to repeat "in Japan".

This year, the record ofor the hottest temperature in Japan was broken, and ithe new record became 41.8℃. This year, the record for the hottest temperature in Japan was broken, and the new record became 41.8℃.

"The new record" is not the same as the one that was broken (which would usually mean the specific measurement, not just the category of record), so better to use "the new record".

And iIt was still hot from June to early September and w. We got biteten by mosquitoes and we were itchy, so we sometimes couldn't sleep well. It was still hot from June to early September. We got bitten by mosquitoes and we were itchy, so we sometimes couldn't sleep well.

"And" at the start of the sentence is generally considered bad style, and isn't needed here. "we got bitten" / "we got bit" are the past tense forms of "bitten". I think "bitten" is more formally correct, but "bit" is also very common. Too many "and" clauses in one sentence sounds a little awkward, so split this into two sentences to be more natural.

However, in the country that I've lived in, there are almostonly two weeks of summer and it's not as hot as summer in Japan. However, in the country that I live in, there are only two weeks of summer and it's not as hot as summer in Japan.

"country that I've lived in" means somewhere you used to live but don't live now. But based on the rest of the piece, I think you still live in that country.

Guess why many people put up thuse umbrellas in Japan in summer even it isn't raining. Guess why many people use umbrellas in Japan in summer even it isn't raining.

I used to go to the Japanese school by walking and it took us 45 minutes and, it was so hot and me and my sister wasere exhausted every day in summer. I used to go to the Japanese school by walking and it took us 45 minutes, it was so hot and me and my sister were exhausted every day in summer.

So we were also puting up theusing umbrellas. So we were also using umbrellas.

tThere are special one just for avoiding sunlight. There are special one just for avoiding sunlight.

It is important to put up umbrellas or other ways to measures to protect against heatstroke. It is important to put up umbrellas or other measures to protect against heatstroke.

Sometimes we heard that old people or children washave died because of heatstroke in the news. Sometimes we heard that old people or children have died because of heatstroke in the news.

"A was Xed" means "X was done to A". You can't use it for intransitive actions like "die". Instead "have died" is the passive form.

So, we had to drink plenty of water and watch out to put in foods in the fridge toso they don't go food bad. So, we had to drink plenty of water and watch out to put in food in the fridge so they don't go bad.

Countable "foods" is like "cuisines" or types of food in the abstract. It's more like a collection of recipes than actual items of food (which uses the uncountable form of food)