Languages of Tyria
The Tyrian language, often referred to as the "common tongue" is spoken in various different dialects by the majority of all sentient races throughout Tyria, allowing people of different nations, cultures and races to effectively communicate with each other, even if their endemic written language systems were different from each other.
The history of the Tyrian language goes back to ancient history, at least to the very last cycle of the Elder Dragons' awakening over 11.000 years ago. In this cycle which served as a prelude to the one currently experienced in Tyria, we know that five races were shepherded by Kralkatorrik's Dragon champion Glint. These races (Dwarves, Jotun, Forgotten, Mursaat, and Seers.) had their own languages, but as they fought and struggled to survive the predations of the Elder Dragons, they realized there would be a need for communication, not only amongst themselves in the short term but to convey information to the future generations of whatever they learned about the Elder Dragons.
It is unclear whether these ancient races adopted one of the races' languages as the common tongue, whether the common tongue already existed and pre-dates those races or whether they devised a new "common" tongue that all the races learned and passed on to future generations and intelligent races they were to end up coming in contact with. Whichever of these theories is correct has been lost to history and what we are left with is simply the knowledge that it was that first cooperation among the races who survived the last Elder Dragon cycle which gave birth to the widespread use of a common language that all Tyrians are raised to learn. It is however safe to say that it is due to the cooperation of these ancient races that we use Tyrian as a common and shared language to be used by basically every sentient race and creature to some extent.
After millenia since the Elder Dragons returned to slumber, it became clear that being able to understand one another was vital for survival, trade and even war. 
- New Krytan
New Krytan is the standard writing system used in Tyria. It was formed to act as a way to aid in sharing knowledge and communication between the various races. It is a hybrid of the Ascalonian and Old Krytan writing styles and was created in 1105 AE by the Durmand Priory. Villem Caraga of the Priory wrote about the creation of this system in Cultural Diffusion in Contemporary Kryta.
- Old Krytan
The alphabet of the Asuran script which can be seen on in-game screens and displays is based on a small subset of their written language converted into a cipher of the common tongue; specifically the letters being derived from how they write mathematics. 
The origin of "Bookah" is that of a large, loud, bellowing, and dumb imaginary creature that the asura used to frighten their children. Since their surfacing, they use the term to refer to those they view as inferior beings, most often humans. Bookah is one of the few known words of the old spoken asuran language known to outsiders.
A term used by asuras to refer to those in command above them, but in a less respectful, more playful, context.
The charr have their own language, but have shown knowledge of common language long before the truce between Kryta and the High Legions. Communication problems might have played a part in the past between human-charr animosity. Instead of a written language, the charr have used ideograms to communicate battle orders and similar information, though their use was not widespread during the rule of Flame Legion's shaman caste.
The Norn also have their own language, both spoken and written. Though this language is fading fast from Norn culture. The spoken language can still be heard in names of places like Hoelbrak. The written language is no longer found, it once could be seen written on sign posts of the Far Shiverpeaks although these were only a few runes, indicating the language to having already been at the stage of being edged out in favor of the common tongue.
Not much is known of the old language of the Norn, except that it has more than one word for 'hunt.' 
- During development of the game, in the blog post that included Cultural Diffusion in Contemporary Kryta, Content Developer Matthew Medina stated that there would be five translatable languages with the release of Guild Wars 2: New Krytan, Ascalonian, Canthan, Asuran Script, and a fifth, previously unseen, language. That fifth language was to be Orrian Syllabary, but was shelved later on during development.
- Arenanet internelly refers to the Tyrian language as "Common" in homage to Jeff Grubb and the original D&D design team who solved this exact problem (Tyrian being a ubiquitous language) years before us. 
- ^ a b c d http://webcache.googleusercontent.com/search?q=cache:https://forum-en.guildwars2.com/forum/game/lore/Lore-Q-A/page/2#post6176083