Common Elvish

The most prominent language in the World is formally known as Common Elvish, but is often just called Elvish for short.
The modern Elvish language is a naturally evolving mixture of diverse, historical cultural languages.
As each elven society began to grow, each had it's own language, often similar to it's neighbors within the same environment, but differing more between environments.
Given the long lifespan of a single elf, traveling elves quickly picked up on languages foreign to their own and began incorporating them into their own language, teaching their friends and society about the words, phrases, and sentence structures they had learned.
Thousands of years later, this has evolved into a singular language spoken by all elves, although some maintain a regional language as well. Additionally, regional dialects and regional slang exist.
While elves in the stories speak Elvish, it is translated into the reader's language so that they can understand what is being said. Untranslated elvish indicates that the phrase or word may be part of an original, regional language that wasn't incorporated into Common Elvish. These will often be accompanied by a footnote to give a best approximate translation.
There is a whole sub-language dedicated to numbers and measurement. Some elves find it humorous when there is homophonic crossover: when someone is describing a number and it sounds like a joke. Many numbers can be expressed in a way that sounds like a sex act, like an Elvish 69 joke.