Writing COMMENTS field on MPEG / ID3v2

[Urs Fleisch]

If nothing different is set, TagLib will set the "language" field of
"COMM" frames to "XXX" which is defined in the standard
https://id3.org/id3v2.4.0-structure (section 4.  ID3v2 frame
overview):

> The three byte language field, present in several frames, is used to
> describe the language of the frame's content, according to ISO-639-2
> [ISO-639-2]. The language should be represented in lower case. If the
> language is not known the string "XXX" should be used.

So this is correct, as is "eng" as set by iTunes, which is the
ISO-639-2 code for English. With the properties interface, there seems
to be no way of setting the "language" field. If you have to do this,
you will have to use the lower level `CommentsFrame` class.