Looking over the tags it seems that there is no clear distinction between ISO 639-1 language codes and ISO 3166-1 country codes.
My understanding is that two-letter codes in tags are supposed to be country codes (language codes are embedded in the file name and subject to translation).
E.g. there are several Austrian events tagged as "de".
P.S.: As a side note, the tag for Great Britain/United Kingdom is "uk" instead of the correct "gb" (which is my fault).