$0.02My thoughts are that maybe because you, like any modern person, want to own stuff - cars, software, lovers - and it actually irks you if the possession is not complete. So you need to invent your own language as you like it.
My second idea is that you try to imitate some person who has created a language, like some sci-fi writer - of whom I unfortunately can't provide an example - or fantasy writer, like Tolkien. Yep, that's a cool world he built, and for that was a cool person. Except, he didn't do it idly (at least I don't think so). First, languages and their formation was a part of his job (he worked in philological research, taught English language and literature), so it probably contributed to his better understanding of his subjects, and passion for profession never harms... I think everybody here knows that Tolkien's world was for him only the background for his languages... My point is, his worldbuilding went hand in hand with his service to the mankind.
I don't want to imply that everybody here is a fulltime slacker, and it's more than likely that you are quite successful in some other, more useful field, but still, why do you have to create your own language, instead of adopting and properly researching some real, but obscure, one? With a real language...
First, it might actually save you some effort.
Second, don't fool yourself - you're never going to make your own language more authentic and complete than any even completely dead real language. Languages with the lowest number of speakers had millions of people "work" on them, historically. Also, naturally and confidently.
Third, and it's a bit sad, I think, - there are enough languages and cultures in the real world which have only hundreds or even dozens of speakers and bearers, and they go extinct at a rate a bit lower than the rate of extinction of biological species (hyperbolic comparison). Well, you might not care, but I think I might have a few points of interest on that matter. I dare to predict that as our human society becomes more uniform, the need to be ethnically tolerant disappears. And if smaller cultures don't suffer for it, then, in case we actually meet an extraterrestrial intelligence, peaceful contact might become harder. My other point is, that if the whole world becomes English-Spanish speaking, there won't be much to base your speculation on, because the majority won't know that any other languages just might exist.
Also you may want to consider that there's actually a lot of fascinating places on non-imaginary Earth, which won't have any qualms with speaking a non-imaginary language, - or places which will soon stop being fascinating. Instead of wracking your brain with a geology not wanting to be authentic, you could study one that already is.
Yeah, of course there are drawbacks. In actual real world there are no furries which aren't a) fat and crazy, or b) atavistic and not sexy at all. A real-world location has absolutely no geek value, so you'll lose a lot of respect in the eyes of your geek friends or even have to find yourself new, non-geek friends. Oh, yes - you won't own the language, and probably won't be able to get the property rights for the land.
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