One thing that has always bugged me about fantasy novels is that in many of them, language barriers are either non-existent or glossed over. While there are many notable exceptions (George R.R. Martin, Robin Hobb and Christopher Paolini come to mind), there are also a number of well known books where language barriers are non-existent despite there being every reason for them to exist.
Let's start with a real world example: Afrikaans is one of the official languages of South Africa. It is the language that was spoken by Dutch settlers who came to South Africa from the 16th century and onwards. From what Wikipedia tells me, the language started diverging somewhere in the 18th century. It is said that Dutch and Afrikaans are still largely mutually intelligible. But as a native Dutch speaker I can tell you that Afrikaans is tough to follow, even if the majority of words are largely identical. I'm sure native Afrikaans speakers will agree.
So this is a language that has been developing independently for about two centuries, and is hard to follow for people used to speaking the parent language. In fantasy novels it is not unheard of for people who have lived in isolation for over a thousand years to still speak largely the same language, with a light accent at most (yes Wheel of Time I am looking at you). This I find completely unbelievable, which is why I've tried to create language barriers in my own novels.
My Hunter in the Dark trilogy features 6 distinct languages. The first is the language spoken in the Seven Villages and Chaedin, which is essentially English. The second and third are introduced in Gift of the Destroyer: Northlandic and Arvayan, the former being an offshoot of the latter. In The Raven's Endgame I also introduce Ktharian and a few bits of the Wanderer language. Finally, my upcoming novel Hunter's Chosen introduces Averri and expands both Ktharian and the Wanderer language.
Each of these languages has its own distinct grammar. Arvayan and Northlandic have fairly simple grammatical rules. Word order is mostly similar to English, and verbs do not change with tense (a feature common in Scandinavian languages I believe). The vocabulary is fairly limited, and differences between the two languages are minimal. The Wanderer language isn't well developed, but is a Verb-Subject-Object language that borrows some words and sounds from Turkish and Arabic. Averri borrows sounds from African languages such as Swahili (at least, how it sounds to me).
The language I have spent the most time on is Ktharian, however. It is easily the most complex of the languages that are featured in my book. Most of the complexity comes from verb grammar - which varies depending on the social status of the speaker, the listener, the tense and subject. Simply said, there are roughly 130 ways to use verbs in Ktharian, and it also has the largest vocabulary of all my construct languages. Incidentally, I had to write my own translator application just to keep track of this language.
It's been a lot of fun working on these languages, and while I feel they add a level of realism to my books, I will probably not be featuring constructed languages in future books. Creating and expanding these languages takes a lot of time, and while I think I have done fairly well in making them, I'm well aware that real languages do not work the way I have constructed them. For the purposes of my books, they seem natural enough, but I am not a linguist, and I feel my time is better spent writing than inventing languages.