I’ve just started to use Mp3Tag for movie files. After entering various texts for Country in over 1,000 files, I had the vague recollection that the Country field may be designed for only a 2-letter Country code rather than free-form text like “France\\United Kingdom\\Czech Republic.”
Do I need to convert all my free-text entries into 2-letter codes? If so, can someone provide a link to the list of the codes?
It really depends on what player you'll be using, and how it reads country metadata (for a library index, for example).
The best way to make sure is always experience: use a sample of your collection (or some test video files), write their country tags in free-form text and see if your player can interpret them.
If it can't, other often used country codes are the 2-letter and 3-letter designations.
And if your player can handle them all without issues, then it's more a matter of personal preference.
Hope this helps.
The codes can be found in ISO-639-2.