I haven't used this site for the past three years because I changed jobs and didn’t need to use English any more.

Last Mmonth, I moved to the HR division.

Our company has a lot of workers from other countries, so it became necessary to use English again.

That's why I came back here.

I’ve been doing overtime everyday since I moved to the HR, but I’ll managetry to post my English journal entries here at least twice a week.

I wish you all have a wonderful new year!

Alternatively: "I hope you all have a wonderful new year!"

Last Mmonth I moved to the HR division.

I’ve been doing overtime everyday since I moved to the HR, but I’ll manage to post my English here at least twice a week.

I haven't used this site for the past three years because I changed jobs andmoved to a job that didn’t need to use English any more.

The original sentence was fine, but this feels like more common phrasing to me.

Last Mmonth I moved to the HR division.

Our company has a lot of workers from other countries*, so it became necessary to use English again.

I feel like more typical phrasing here might either be "foreign workers" (workers in Japan, but not Japanese national citizens), or "overseas workers" (workers outside of Japan who speak English).

I’ve been doing overtime everyday since I moved to the HR, but I’ll manage to post my Englishtry to write here at least twice a week.

Manage -> try: Since you cannot foresee the future, you cannot know what you will manage.

Looking at the Japanese version:
> なんとか週に2回は投稿するようにします。
するようにする is commonly thought by English speakers to mean "try," "endeavour," or "make sure that/to."

Another wording you could try is "I think I'll be able to manage posting here at least twice a week."

I haven't used this site for the past three years because I changed jobs and didn’t need to use English any more / anymore.

Last Mmonth I moved to the HR division.

Our company has a lot of workers from other countries, so it became necessary (for me) to use English again.

more natural: …so I need to use English again.

That's why I [came | have come] back here.

I’ve been doing overtime every day since I moved to the HR, but I’ll manage to post [my | in] English here at least twice a week.

In this context you need to use “every day” (two words).

I wish you all have a wonderful new year!

or: I hope you all have a wonderful new year!

Last Mmonth I moved to the HR division.

I’ve been doing overtime everyday since I moved to the HR, but I’ll manage to post my English here at least twice a week.