Thanks for the suggestion.

I created a small test spreadsheet with two entries:

Fundación

ФОРСУНОК

The Encoding method returns 1 (8bit ASCII or single byte UTF-16) for the Spanish text and 2 (UTF-16BE) for the Russian text.

I also modified the TextFmt routine in FmtDefault.pm to print the value of the parameter $sCode. It was undef for the Spanish text and UTF16-BE for the Russian text. So the routine just returns the Spanish text since $sCode is undef, but formats the Russian text (which gets mangled) as UTF16-BE.

sub TextFmt($$;$) {
    my($oThis, $sTxt, $sCode) =@_;
    if((! defined($sCode)) || ($sCode eq '_native_')) {
        print STDERR "$sTxt/sCode " . (defined($sCode) ? "is _native_"
+ : "undefined") . " - returning text\n";
        return $sTxt;
    };
    
    # Handle utf8 strings in newer perls.
    if ($] >= 5.008) {
        require Encode;
        print STDERR "$sTxt/$sCode; returning text with UTF-16BE encod
+ing\n";
        return Encode::decode("UTF-16BE", $sTxt);
    }
    
    print STDERR "$sTxt/$sCode; formatting with pack/unpack\n";
    return pack('U*', unpack('n*', $sTxt));
    #return pack('C*', unpack('n*', $sTxt));
}
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The Encoding method returns 1 (8bit ASCII or single byte UTF-16) for the Spanish text

I don't know where that description is coming from, but it suggests a serious misunderstanding of the terms being used.

You can say "single-byte ASCII" (which is redundant, since ASCII by definition uses only 7 bits), but it's strange to say "8bit ASCII", because ASCII does not refer to values in the 0x80-0xFF range, and people usually speak of "8-bit characters" as being in contrast to ASCII (because 8-bit characters are the ones in the range 0x80-0xFF).

Saying "single byte UTF16" is simply nonsensical. It's an oxymoron.

I copied the text in parentheses from the page you linked to, which explains the encoding method's return values:

The encoding() method returns one of the following values:

    * 0: Unknown format. This shouldn't happen. In the default case th
+e format should be 1.
    * 1: 8bit ASCII or single byte UTF-16. This indicates that the cha
+racters are encoded in a single byte. In Excel 95 and earlier This us
+ually meant ASCII or an international variant. In Excel 97 it refers 
+to a compressed UTF-16 character string where all of the high order b
+ytes are 0 and are omitted to save space.
    * 2: UTF-16BE.
    * 3: Native encoding. In Excel 95 and earlier this encoding was us
+ed to represent multi-byte character encodings such as SJIS.
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