The book I plan on writing will contain sentences (mainly dialog) in Elvish (which I shall invent myself), and I was wondering if I should display: 1. Under the text of the page: The elf uttered a few words: "Porono kiay pokono pankaka"* *Do we have any pancakes? 2. Or should I have a mini dictionary at the end of a box, à la "Eragon" (for those who have read it)
Me again Inventing a language is a huge undertaking, but if you don't do it at the start, you'll struggle to do it retrospectively. Remember that Tolkien was actually a Professor of Old English, which is as foreign a language as Elvish would be (example sentence: "wyrd biþ ful aræd!"). So, he knew his stuff before he started making his own up. To be honest, if you have your heart set on Elvish, I'd just use a few rare bits for atmosphere, where the meaning is either obvious from context or else doesn't really matter. Actually having to consult footnotes or a glossary to follow the story will get very old very fast.
Ok, I shall endeavour to only use it when it matters not, or with spells (or i suppose I should call "magical incantations")
I agree with what fb has to say. You need to prepare a solid plot line first and then venture into bringing a new language to life if it's necessary.