Different programs require ID3v2 comments to be written with specific language code identifiers. Mp3tag's default is 'eng', but depending on the language of your Windows installation, you might want to use the ISO-639-2 of this language so that comments are displayed in Windows Explorer or Windows Media Player (e.g., 'deu' for a German Windows installation). However, if you use another language code than 'eng' iTunes won't read the comments of your ID3v2 tagged files anymore, since it requires them to be written with this particular language code identifier.
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