Why bir in this sentence?
You must use "bir" as a article-like thing if you have an general noun with an adjective :)
Yorum is continuing so it should be i am using not i use
Is there a difference between saying formal language and an official language (of a country)?
Yes, English is the official language of the UK, but if I called you an #@se-hole that would not be formal!
I said 'in the office I use formal language' and got the following Duolish gibberish...
You need the article "a" here.
In the office, I use a formal language.
"Ofiste resmi bir dil kullanıyorum."
Translation: I use formal language in the office.
I am using formal language at the office. - Correct.