The One Language.....that of Fiction Fantasy.....
In the many novels that we read, we often enounter circumstances where the author has the character speak in strange tongues, and this is taken to be the character's native or ancient language. Take Dragonlance, for example. The denizens there all speak a common language, that of Common. However, each race has its own language, and each race, if further sub-divided into provinces, has its own language too. So, the elves have their own language, Elven, and the elves of Qualinesti will speak that tongue, as well as their own Qualinesti tongue.
However, in other novels, where the characters are all of the same race, sometimes the author has them come from various countries, such as in the case of Heralds, where they speak a common trading tongue, and each country will have its own language, Karse has Karsite, the people of Valdemar have their own tongue, and so on. Also, the author sometimes might feel like making the story special, so they will invent a new language, such as the Shin'a'in, Kaled'a'in and Tayledras tongue.
And here's one of our favourites: In Dragon Prince / Dragon Star, the characters there have an older language, the Old Tongue, which they use in names and to term certain material. Made up of root verbs that join together to form new words, it really is very interesting to look at.
So you see, the language of Fiction Fantasy is an abstract and diversified one. There are many versions of it, for every series has a different type of language, but they are all the same, for it is all just a way for the characters to communicate what they wish to in a special, unique form.