I've always seen the Vithelans as Pacific Islanders - both Polynesian and also Malay/Javanese/Balinese/etc. Vormain is similar to Japan in its Closed era.
Fonrit is North Africa, similar to the Barbary States or Mameluke Egypt (and also the Ottoman Empire).
The Lunars worship a goddess of mirrors and masks, so they appear different each time you look at them.
They have aspects of Darius' Persian empire, the Romans, and even 19th century Britain and the 20th century Soviet Union... and in their early days they strongly resemble Islam during its expansion era in the 7th-11th centuries.
My first instinct is to generate a seperate tribal identity for each - but they may be from a single stock, and thus share common believes, language and customs. Is that reasonable to assume in a region such as the far NW of Genertela? And are they of Orlanthi dencent?
G:2A describes several Fronelan tribes specifically as 'Orlanthi'; I'd assume that others are too, but not all of them. The Malkioni settlers in Loskalm are presumably entirely different in culture, language and race; the Galininni horse people and Rathori bear-worshippers are also separate. Of course, the EWF and God Learners will be spreading their own language and customs even outside their formal borders; this is an age of cultural assimilation.
I'd assume that two Orlanthi tribes will be similar but not identical - they might share a common language, there may be differences equal to those between British and American English, or they may be as dissimilar as English and Dutch, or English and Romanian (both Indo-European languages). Likewise for religion; all tribes will have some unique gods of their own, but very distant tribes might follow completely different rites. That's what the Godlearners do, after all; work out that the god Abcd worshipped by Tribe A is actually the same divine being as the god Zyxw worshipped by tribe Z.
More like the Parthians/Persians according to Greg.