One problem of writing fantasy is the language (yes, I hear you, you writers of historicals).
I'm always very careful with the language I use, even while just typing along - you all know how hard it is to find mistakes afterwards. Sometimes I put expressions into brackets to be replaced by the respective expression in my fantasy languages. So, you'll never read someone saying "Oh my Gosh!" or "Get your a** moving!" :) Although Ella is quite flippant at times, I always have to be very careful with her flippancy.
Or I use those brackets when I cannot think of a better expression, but feel that it is "too much my language". (One example is "draw the short(er) straw". In German that sounds too... I don't know, but not fitting. [Although the German expression says mostly the same, drawing the shorter one. And straw would fit with one of the people. Hm... maybe I'll make that a direct translation?] Or would they say "stew"? That sounds less silly to my ears in English than it does in German... *d..n*)
But how far can one go? How much do I replace with the Fantasylanguage? How far do I stay away from casual language and use the more formal form?
Their language pattern discerns the characters and I do not want them all to sound like they had a stick in their rear.
But of course the reader has to understand what I'm saying, he or she only knows "our" language. I cannot write a book in a language that I invented. :)
ETA: Strangely, whenever Ella speaks to me (although she's usually quite insolent towards me) I know exactly that she's thinking in her own language and her speech is coloured by it. I just cannot. get. it. on. paper.