Made up words

I was thinking about made up words the other day. My current MS is in Portuguese, but I intend to translate it later and, in fact, think in Portuguese and English while writing it. I made up a word in English to name one of the places in my MS and even though I know the word doesn't exist, I want to use it. I was wondering if that would be perceived as a mistake (since I'm not a native English speaker) or if people will understand what I mean. FYI, the word is southest, as in, the one further south.
Can a native English speaker help me with this? Is there a better word for that, or is it okay for me to use the made up one? Can you understand what I meant without the explanation?
Thanks a lot in advance!

Thanks everyone! I'll look into 'southernmost' when it's time to translate to see how it fits.
The name is not in a dialogue (at first, though it appears more than once), but the narrative is first person. Also, it's a made up world and the name aplies to a settlement and would appear in caps. The problem is really the similarity with southeast and southwest (it could be seen as a typo).
SaraP, in Portuguese I'm writing it as Sulista. It is not what I mean but, like I said, I think in Portuguese and English and don't really bother translating my thoughts to Portuguese accurately, since the final MS will be in English.
I think I might change some of the other settlements names to made up words to make it more clear, but this still worries me, because the so called Southest Settlement appears in the first chapter, while the others are only referred to later on (way later on).