After making sure a string was not interned, ( "Symbol findInterned: 'test123'" returned nil ), I did
| s | s := #TEST123. s translateToLowercase. s -> (Symbol findInterned: s)
and the resulting s is a Symbol and it is not interned. To make sure that the primitive is invoked (and not its fallback code), put a halt in ByteString class >> #translate:from:to:table: