Depends on your definition. If you wanted to go by the first listed, which is "a line or border separating two countries," you might try junaar-grind. If you wanted to get more interpretive and into the psychology behind the dov, frodunslaad, as in "endless battlefield," might work, implying a constant fight for more power.

In the more commonly used sense, which is "the extreme limit of settled land beyond which lies wilderness," you might say pindaarsenid, plains of nothing. You could describe the idea of a frontier with a phrase, like lokke vomindok (unfamiliar skies), or qalossehofkah oblaan (end of the influence of civilization), or something like that. 

Hope that helps!