I am still in the process of working out the quirks of the languages in Evandar. There is a group whose linguistic background is developed out of Anglo-Saxon. I'm not calling them Anglo-Saxons because it isn't set in our world. That doesn't change, however, that the oldest language for the principle region where the action of the first few books is Anglo-Saxon. I have been using a translator/dictionary that I found online to work up phrases to use in the stories.
I want to have the evolution of the languages reflect the passage of time, thus there will be some who are geographically isolated who use a more archaic version and others that use a more 'modern' version with a greater variety of loan words from other languages to reflect the more cosmopolitan area they are in. The border cities, like Farden, are going to have something of a polyglot going on. I haven't really done much with that yet but that is because I'm literally still working on developing languages.
I was inspired by J. R. R. Tolkien and it shines through in quite a few areas. It was Tolkien who introduced me to the idea of generating languages for your fantasy world. I am not a linguist by any stretch of the imagination. Where as Tolkien had a great deal of study and expertise in how languages work, I am learning as I go along. Right now, I am focusing more on the roots of the English language because I am using it as the primary language of Evandar (with regional variations). Also, because I am most familiar with English since it is my mother tongue.
I think the language I am going to have the most difficulty in developing things based off of it will be Latin. I have no real idea what is going on with that language and I don't know anything beyond a few phrases and the names of plants. I am pretty intimidated by Latin but I know with some careful effort, I will get at least a minimum grasp on how words are structured so that I can develop my own Latin-esque words. I know that some people will jump to the conclusion that I am attempting to steal the idea from J. K. Rowling but that is not where it is rooted.
I have a region that is based off of the Roman Empire and I want the language and customs to reflect this. I am planning to eventually write something set in that region to reflect some of the clashes that happen between them and the peoples around them. The military history and the religious history of the Roman Empire has always fascinated me and I figured I could apply some of that towards my writing. That is the sort of thing that sparks ideas for me.
I'm attempting to build languages based off historical languages in our world. I will be inventing one or two whole cloth but those are going to be the exception. They're not going to feature heavily until much later into the series. And when these languages pop up, they will have a glossary of terms at the end of the book, so that you, the reader, can make sense of what the characters are saying. Because it never struck me as fair to leave the readers to guess the meaning of terms on the basis of context alone. You wind up unsure if your definition matches the one intended by the author and I personally find it a really awkward thing. It is my promise that I will not do that to you.