First of all, welcome to Taijitu!
I don't know much about where mountains or plains speciifcally are - that's more St Oz's realm, as he's the cartographer - but I can briefly give you an overview of some temperate areas and their cultures.
Phoenixia, in the southwest, has the countries of New Delfos and La Llanura Libre. Delfos is somewhat Old World in flavor but he'd be able to tell you more about his nation specifically. La Llanura Libre is somewhat Latin American inspired; it's a former Myrorian colony that waged a war of independence. Again, he'd be able to tell you more but from what I've gathered it's a mostly-democratic nation in modern times with some corruption problems.
Northern Cefnor, near the middle of the map, is the other major active temperate area. Cefnor is the enormous gulf surrounded on nearly all sides by land. Starting at the top and going clockwise:
Rykkovaa is a fascist dictatorship that's rather Soviet Bloc - not communist, but not 'what if Nazi Germany won the war' either. Corruption and state bureaucracy is the norm. It's very cold, mostly in the Arctic Circle, and characterized by fjords and taiga in the south and tundra in the north.
Myroria is an elective monarchy characterized by bitter, partisan politics and large expanses of rural taiga. In modern times it transitions mostly-peacefully to a democratic constitutional monarchy. It has close ties to Letonna and Eluvatar. It's developed along the coasts, but the rural interior suffers from a crippling lack of infrastructure and a constantly-flowing brain drain.
Letonna is an absolute monarchy characterized by bureaucracy and a government that brutally stamps out any opposition - especially from minorities. In modern times, it too begins to transition to democracy, trying to come to terms with its history at the same time. Its large nation has a variety of climates, changing from taiga and steppe as you move south to more temperate climes.
Bustos is a nation run as a megacorporation ruled by the Bustos family. Essentially feudalistic, the incredibly wealthy Bustos family owns all land in the country and rents it out to the highest bidder. As far as I know, its climate is pretty temperate.
St. Oz is a communist theocracy that's the world's only matriarchy. Much like women in most other nations before the 1960s-80s, men in Ozia are second class citizens and don't have the right to vote until the 1970s. It functions mostly as a republic, but is a single-party state I think. In modern times, its economic strength makes it similar to China. The north of Ozia and its border with Myroria is very mountainous. The rest of its climate is somewhat of a mystery to me, but if I had to guess I'd say the southwestern coast is Mediterranean.
Eluvatar is a constitutional monarchy sans monarch. Its king disappeared in the 1700s and since then a complex legal fiction arose to justify his lack of successor because reasons. It has close ties with Rykkovaa and Myroria. It's democratic and pretty much a modern Western country. Due to its size, Eluvatar's climate ranges widely as well from the south to the north.
I skipped some inactive nations scattered along the same area there. If I had to guess, I'd say south of Oz might be your best bet. Or maybe ask Oz to replace Slavnayaboda with your nation, as they haven't been active in a while. Phoenixia might also be a good choice - maybe near Delfos, between it and La Llanura Libre.
