Once again Brian's method is simple and fast,
although unfortunately not automatic.
But the premise of linking the [Country] field to the [Composer] field falls down a bit if a track lists more that one Composer.
[Composer] is a list type field, and so is allowed to have more than one value in it, separated by semi-colons. I certainly have a few Classical tracks that list more than one Composer, so whose Country would you put into [Country]? The [Country] field is also a list type field, so I guess you could list the Country of each Composer in the same sequence as the [Composer] field. But that does get a bit messy.
It is a shame that MC doesn't have separate tables for things such as Composer, so that all the details of a Composer could be recorded once, and then just linked to again the track using the Composer's name. But alas, MC works based solely on tags in or assigned to files, with only one record per file.
The closest thing MC has is relational fields, but they only work with respect to Artist, Album, or Series, and they wouldn't work with List type fields anyway. At least I don't think they would, although interestingly the [Artist] field is a List type field, so maybe they do? Relational fields are all one-to-one relationships, such as one Series has one and only one [TheTVDB Series ID]. If the [Composer] and [Country] fields were just String type fields this would be a perfect application for a relational field. I have used a custom relational field to flag a whole series as a favourite, with one click. It works very well.
Note that as there is only one standard [Country] field I would be tempted to add a new field called perhaps [ComCountry] for Composer Country. That way if I ever wished to store the country of origin of the Conductor, for example, I could just create [ConCountry] for that.
If it was possible, I would then make [ComCountry] relational; One [ComCountry] for each [Composer]. But alas that isn't possible.
Anyway, I'm just rambling a bit since this question caught my attention. Carry on.