1

Create a new swift Cocoa App in Xcode 10 with an accented character in the app name, i.e. "Belzebù".
Then the app fails at launch with these runtime errors:
"Belzebù[22477:5859551] Unknown class _TtC8Belzebù11AppDelegate in Interface Builder file at path..."
"Belzebù[22477:5859551] Failed to connect (window) outlet from (NSObject) to (NSWindow): missing setter or instance variable"

Reverting to the legacy build system solves the issue.

P.S. I do not know if it is correct to report this bug here.

2

Swift bugs can be reported here: http://bugs.swift.org
For Xcode, see here: Bug Reporting - Apple Developer

I suspect the fault for this one lies partly in each, so I would report it to both.