It also depends on where you are looking. In the database it is using the three character strings. But, when the language is put into the OPF of an ePub, the format needed there is used. For English, that will be "en" as calibre does not distinguish between the likes of "en-UK" and "en-US".
And in case you haven't guessed, I don't know what standard is being used. My language needs are usually simple. If I need to know something else, I always look it up at the time.
But the question that comes to mind is: What are you doing that it is important?
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