It was very happy that I could see many people in Japan.

英訳に間違いが有れば正してください。

日本で多くの人に会えてとても楽しかった。
It was very happy that I could see many people in Japan.

答え
人が主語なのでIt was very happyはおかしい。
I was very happy that…
I was very happy to…
にする

I was very happy to see many people in Japan.
日本でいろいろな人と会えて嬉しかった。
It was good that I could see many people in Japan.
日本でいろいろな人と会えてよかった。

happyは人が主語