When you toon transfer, and have a character with an ascii name... or make a toon with your existing name, which forces a name change on the new realm...
The ascii character cannot be used for the new arrival, or in the case of the initially chosen name when you needed to change it.

However, once the name filter has rejected your first choice of a name...
You can change the name to anything other than what had initially been rejected.

So if you want: äpple.
You would enter: äää.

The äää is rejected, because of the ascii characters.
It prompts you for a new name, which will bypass the filter.
You enter äpple, which it accepts.
Enter the world once, and you wear the rejected name.
But on your next log in, you'll have the accepted (second) name.