Looks like Split() is not case sensitive - I just checked with 4.21 and 4.50.

This version using a different Replace seems to work ok, and you don't need to switch case sensitivity on:
Code:

$NAME = "Ändern,Wäsche,Überraschung,überwintern"
 
$NAME = replace($NAME, "Ä", "Ae")
$NAME = replace($NAME, "Ö", "Oe")
$NAME = replace($NAME, "Ü", "Ue")
$NAME = replace($NAME, "ß", "ss")
$NAME = replace($NAME, "ä", "ae")
$NAME = replace($NAME, "ö", "oe")
$NAME = replace($NAME, "ü", "ue")
$NAME = replace($NAME, " ", "_")
 
? $NAME
 
Function Replace($String,$SS,$RS)
	While $String
		If Left($String,1)==$SS
			$Replace=CStr($Replace)+$RS
		Else
			$Replace=CStr($Replace)+Left($String,1)
		EndIf
		$String=SubStr($String,2)
	Loop
EndFunction