The phrase "I have confirmed" is correct and usable in written English. You can use it to indicate that you have done something to ensure that something is true. For example, "I have confirmed that the meeting will take place at 9am tomorrow morning.".
"I have confirmed myself that CIA planes landed in Poland..
I have confirmed with the campaign as late as 3 p.m. this afternoon".
"I have confirmed that this was a heavy and deadly bombardment," he said.
"I have confirmed with his lawyers that our offer of consular assistance stands should he require it".
"I've confirmed that the letter was delivered to them".
And I've confirmed that agreement with the Eugene O'Neill estate".
It also claimed I had confirmed acceptance of the fare – but I hadn't.
Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.
Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com