To state the problem explicitly, internal storage format of strings doesn't mean you can handle all data streams seamlessly. Internal consistency is fine, but ultimately your program will have I/O.

If I open an ISO-8859-15 or UCS-2 file in a Tcl program, how is this handled? Does it seamlessly detect encoding and translate to UTF-8 or is there more to this issue than what's stated in the article? Also, are the strings stored in a canonicalised form? If not, how simple is it to perform normalisation?

I brought up Perl because it's my experience of a language with excellent encoding support and helps make the point that the issue is probably more complex than just how strings are stored and manipulated internally.