I want to portray the Umathings as a conservative, paranoid lot who live out in the sticks, and who are actually content with their situation because they want to have as little interaction with outsiders as possible.
Remember— These Slontan Orlanth worshippers didn't voluntarily come to Umathela as settlers by ship; they were magically transplanted from Slontos to Umathela by the God Learners! This is the kind of traumatic experience whose victims are bound to suffer some kind of mental relapse later on...
What is the Umathings' world view? City dwellers are sorcerous madmen. Elves and goblins are evil wort-men. Timinits are yet eviller bug-men. Anything else is just monsters. So basically far from home = danger and monsters. As a consequence, the Umathings lead simple and secluded lives. They have their steads, they practise slash-and-burn agriculture, and they raise their animals. They worship their storm and agricultural gods, which are completely unrelated to their neighbours'. I also assume that their language has kept some archaisms from Slontos; I will try to make that apparent in my posts.
However, despite this heavy conservatism and general isolationism, some Umathings are born with an urge to discover what lays past yonder hills... these people are the stuff adventurers are made of.