I started to use English at work

There was one happy thing that happened today.

I sent e-mails to our customers in Indonesia in English.

Indonesia is a country so you'll have to specify who you're sending the emails to. They could be customers, business partners, acquaintances, etc.

In that job, I sometimes get e-mails from customers from Indonesia.

But, a few days ago, I got an email from a customer from Indonesia with a question.

Though I used a translation site ¶
, I was able to send non-fixed form text.

I start to useed using English at work

There was oneA happy thing happened today.

I sent e-mails to Indonesia in English.

I took over a new job from a retiree.

In that job, I sometimes getreceive e-mails from Indonesia.

Mostly, I just receive fixed form text e-mailsthese in a fixed text form.

But, a few days ago, I got an email from Indonesia withhich included a question.

Though I used a translation site ¶
, I was able to send a non-fixed form textstyle of email.

I started to use English at work

Or, I began to use English at work

There was one happy thing today.

This sentence is correct but it would be more colloquial to say "There was one nice thing today."

I sent e-mails to Indonesia in English.

I took overstarted a new job from, replacing a retiree.

In that job, I sometimes get e-mails from Indonesia.

Mostly, I just receive fixed form textorm letters. (Or, formulaic e-mails.)

But, a few days ago, I got an email from Indonesia with a question.

Though I used a translation site ¶
, I was able to send non-fixed form textwrite a custom response.

I yearn to use English at work, but I am afraid of making mistakes and being rude.

I don't know if I've ever used "yearn." It's kind of old fashioned or too emotional for most contexts.
For casual writing, I would just say, "I really want to (use English at work)."
You could also say:
I long to use English
I hope to use English

I start to useusing English at work

or 'I started to use English at work'

There was one happy thing today.

not necessarily incorrect but its more natural to say 'Something good happened today'

I sent e-mails to Indonesia in English.

In that job, I sometimes get e-mails from Indonesia.

Mostly, I just receive fixed form text e-mails.

'fixed form' is uncommon, at least where i live. you can use 'plain' or 'standard' instead.

But, a few days ago, I got an email from Indonesia with a question.

TAlthough I used a translation site ¶
,I was able to send non-fixed form text.