How can I get a list of all possible attributes that the API provides?

All profile attribute values such as languages, dialects, etc., can be retrieved via the attribute endpoint (with the exception of countries and states, as these are standardized lists, some of which are very extensive).

For countries, ISO-3166-1-Alpha-2 codes are used: https://de.wikipedia.org/wiki/ISO_3166-2. For states/federal states, ISO-3166-2 codes are used, e.g., for Germany: https://de.wikipedia.org/wiki/ISO_3166-2:DE.