Understood, and you are correct, but the id3v2.3/4 standard specifies that language codes should be represented ISO-639-2 three character language codes (id3[b]_dot_[/b]org/d3v2.3.0 see the third last paragraph of point 3.3), which in turn recommends language codes to be lowercase, though they should be considered case-insensitive (www.loc[b]_dot_[/b]gov/standards/iso639-2/faq.html#21), so technically uppercase should be OK but practically it would be more compatible with other software if it was lowercase, just like the ISO-639-2 list itself.
in fact, using [b]und[/b] (undetermined) might actually be the most correct approach as XXX is not a valid language code?

That's just my two cents worth, I understand that changing it is complex and brings with it a host of challenges...

Hi,
they should not be case sensitive. From my tests MM recognize both.

I have seen a few topics on the XXX language code used on the Custom Fields COMM frames.
Whilst I understand the reasoning behind using XXX, would it be possible to use lower case letters xxx instead, as uppercase letters are not compliant?