2nd day.

Hi. I was surprised that so many friends had sent me messages.

Thanks a lot. I'm glad,though I haven't read them all ( messages) yet...sorry.

It's getting hotter and hotter in Japan.
How about your country?

Bye.

A good start.  As you have mentioned 'messages' in your opening sentence, there is no need to repeat it - can just say 'them'. 

2nd day.

Hi. I was surprised that many friends sent me messages. (I was surprised to receive messages from so many people.)

Thanks a lot. I'm glad, although i haven't read them all yet... sorry (for that).

It's getting hotter and hotter in Japan. (The weather is getting hotter and hotter in Japan.)
How about your country?

Bye.

Note: Excellent! I tried to give you alternatives to show you how to say the same thing a little bit more naturally. But your text is perfectly all right as it is. Congratulations!

2nd day.

Hi. I was surprised that many friends so many people sent me messages.

Thanks a lot. I'm glad,though i haven't read all messages yet...sorry.

It's getting hotter and hotter in Japan.
How about your country?

Bye.
______

It's getting more rainy in the UK. And cold, at the same time. You have no idea how much I'd love to be in Japan right now.