Change from ISO-8859-1 to UTF-16

[ohrenkino]

See also here:

Problems with ID3v2 tags in Unicode format?

Since version 2.33 Mp3tag supports Unicode through the whole program. Mp3tag writes ID3v2 tags in Unicode format (UTF-16) by default now, but there are still some programs, portables or car radios which have problems with ID3v2 tags stored in Unicode format (though this is specified within the ID3v2 standard). To use Mp3tag with these programs or devices, simply enable the option (x) ID3v2.3 ISO-8859-1 at [:mt_options:] Options > Tags > Mpeg. After that, just save the tags again by clicking […

So, in fact, it is more likely that older programs and devices have problems with UTF-16.
UTF-8 is used for ID3V2.4 which is probably even more recent than ID3V2.3 and UTF-16.

To transform the tag encoding
Set the options to write UTF-16 tags.
load the files
select the files
press Ctrl-S to save the tags again.
Done.