What’s wrong with just writing it as Mikolaj? It’s not like it’s a kanji or something.
So the solution is for the user to change their entire windows account name, rather than handling common characters in your code?
Changing all software to respect their perfectly valid name isn't something they can do.
They shouldn't need to change their name, but if they do, they can ignore all the broken software and go about their day.
This particular user is more capable than most, and found a workaround for this particular problem, which is good... But this is not likely to be the last of the problems.