Fixes #3517

1. Replaces pycountry's generic languages.lookup call with several languages.get(attr=language) calls, where attr is alpha_2, alpha_3, bibliographic and name, in this order. Other attributes are probably not relevant for Streamlink. .get calls don't raise and just return None if the lookup fails. The order of attribute lookups prevents issue utils.l10n: fix pycountry language lookup #3057, where language.lookup("en") would return the "En" language instead of "English".
2. To prevent the AttributeError from being raised when certain optional language attributes are missing, like alpha_2 for example, those will be read via getattr with an empty string as fallback (instead of None), which matches the data representation of iso639. alpha_3 and name attributes always exist.

curl -sSL 'https://raw.githubusercontent.com/flyingcircusio/pycountry/20.7.3/src/pycountry/databases/iso639-3.json' \
  | jq '.["639-3"][] | select(.alpha_3 == "des")'

{
  "alpha_3": "des",
  "name": "Desano",
  "scope": "I",
  "type": "L"
}