How can I get a list of all possible attributes that the API outputs?
All profile attribute values such as languages, dialects etc. can be retrieved via the attribute endpoint (with the exception of country and federal states, as these are standardized lists (some of which are very extensive).
ISO-3166-1-Alpha-2 codes are used for countries: https://de.wikipedia.org/wiki/ISO_3166-2. ISO-3166-2 codes are used for federal states/provinces, e.g. for Germany: https://de.wikipedia.org/wiki/ISO_3166-2:DE.