TYPO3 Core

Observed behaviour:
When a username like 'dörthe' is entered, the user will be created with the username 'drthe' instead. No warning or error message will reveal this to the user.
In the TYPO3 Backend there is no problem when a username with umlauts in it is created; it just works as expected.

Expected:
There should be one of the following:
- a warning, that the username is invalid or not possible via the Install Tool
- an error that the username could not be created as requested
- the username is accepted and set correctly