YOU SAID:

i am leaving for Japan next month

INTO JAPANESE

私は来月日本に出発します

BACK INTO ENGLISH

I will leave for Japan next month.

INTO JAPANESE

私は来月、日本に出発する予定です。

BACK INTO ENGLISH

I am leaving for Japan next month.

INTO JAPANESE

私は来月に日本へ出発します。

BACK INTO ENGLISH

I am leaving for Japan next month.