report_file_errno downcases non-english text incorrectly

[Mario Lang]

Hi.

The logic in report_file_errno in src/fileio.c is subtle broken when it
comes to foreign language errors: In particular, the german translation
of "No such file or directory", which is "Datei oder Verzeichnis nicht
gefunden" should *not* be downcased, because "Datei" always has to start
with a capital letter, no matter where it is.

So this logic, is lacking:

        /* System error messages are capitalized.  Downcase the initial
           unless it is followed by a slash.  (The slash case caters to
           error messages that begin with "I/O" or, in German, "E/A".)
           */
        if (STRING_MULTIBYTE (errstring)
            && ! EQ (Faref (errstring, make_number (1)), make_number('/')))
          {
            int c;

            str = SSDATA (errstring);
            c = STRING_CHAR ((unsigned char *) str);
            Faset (errstring, make_number (0), make_number (downcase
            (c)));
          }

Does anyone have a suggestion if we can improve on this?

-- 
CYa,
  ⡍⠁⠗⠊⠕