Language /
Polish was chosen the HARDEST LANGUAGE in the world to learn... :D [1558]
mieszkam w Bieszczadach, w Tatrach, w Dolinie Dunajca etc
It is because it should always be 'w' when used with the names of mountains; 'w' will always be used with the noun 'dolina'; it is 'w górach', 'w dolinie', you will never say: 'na Bieszczadach, na Tatrach, na Andach, na Pirenejach, na Himalajach'.
w Mołdawi instead of na Mołdawi
For the medival tribes of Polans or Vistulans 'Mołdawia" was as far as Bulgaria or France or Bavaria, so they would have never treated it in the terms of 'a well known area in the vicinity', they would have applied 'w' naturally, even if the name was known or popular with them which I doubt.
na Wybrzeżu Kości Słoniowej” originally a territory described by the sailors of old
It is becasue it should be 'na' with 'wybrzeże', regardless if it's a region near a sea-line (na Lazurowym Wybrzeżu) or a country or anything else. The thing is somewhat similar to the phenomenon in German where you say
das Maedchen and not
die Maedchen, although the person is a girl, but the -
chen ending matters more than the sex of the person. However, although the term 'w Wybrzeżu Kości Słoniowej' sounds a little awkward, it is used these days along with the term 'na Wybrzeżu Kości Słoniowej', possibly to underline that we talk of it in terms of a state.