Ooooooh... language! I love language!
If you're going for a "dictionary" for the language, forget it. That'll take you decades to produce. Dictionaries are based on the linguistic corpus, not the other way round. So don't worry about what kinds of words would be "common" in this language; just use what you need to.
You probably want, first and foremost, a program that can create wordlists. There are a few for DF (I use DFLang). Come up with a sample of words, the more the better, and the program should figure out some new combinations. These wordlists are good as a base, but refine them as you need to (for instance, if the word it comes up with for "to be" is five syllables long, cut off four syllables and use the one you like best).
Another thing I like to do: find a sample of English text and "translate" it into your language, coming up with new words as you need to. This will also let you play around with grammar rules.
Then you can branch out into actually writing things in the language, again coming up with words as you need them.
Remember: you want your language to have a specific flavor. Figure out which sounds and combinations of sounds best provide the flavor you want. I always advise looking at the
IPA Wikipedia page, especially if you want some more exotic sounds (although you'll probably stick closer to English).
It really helps to know a bit about language in general, and it especially helps to know other languages: you get more of an appreciation for the "flavor" of language and how varied it can be.
Also be aware that language shapes culture as much as culture shapes language. The way you communicate has everything to do with the way you see the world and your place in it.
I'd be glad to offer any more assistance if you'd like. As I said, I love language.
How exactly is this language going to be used in-game?
EDIT: found
this. I didn't read all of it, but it looks like a good intro to linguistics/conlangs. Probably better than I could give you.