TYPO3 Core

// strip everything non-ascii
$meaningfulPrefix = preg_replace('/[^A-Za-z0-9_-]/', '', trim($meaningfulPrefix));

This is how special characters are simply stripped out. The should be converted using t3lib_cs->specCharsToASCII()

I also had an issue with Images that were not showing up in FE, when they had Umlauts in the title. I could not perfectly reproduce and investigate this issue, however it is completely gone when I switched to t3lib_cs->specCharsToASCII()
(issue imported from #M15466)