"Humanity" is the term associated with the Human race, often used as a unifier between the main and lesser nations. However; Humanity differs from races such as Tauren in that they do not have a "Main" ethnicity, rather many scattered countries and cultures that range from similar to utterly opposite.
First born from cursed Vrykul children, the roots for most Humans is in one of the original main Kingdoms -- namely Arathor. After this point, with influence from what parents went with them, the strongest among them began to make numerous tribes and confederations therein. However; this would come to a rapid end once the dawn of the Arathorian Empire began. After it was declared, these Humans (Who could be best described at this time as barbarians) soon found themselves incorporated or dead. Likewise, confederations were largely dissolved and tribes existed only in terms of ethnicity.
Once a monarchy was solidified and Arathi unified, Imperial goals quickly set on new territory, especially after the Troll Wars. Armed with magical ability, the territory of Hillsbrad became one of the first areas to be in the Empire's sphere of influence. From there on, the entirety of Northern Azeroth (Eastern Kingdoms) and the main islands of Kul Tiras were also subsumed into the quickly growing Human nation. However; as it grew, so too did differences and discontent. The original language of Old Stromic and the manufactored "Universal language" of Old Common (Which would evolve into "New Stromic"/ modern Common) merged in places such as Lordaeron, and with that came cultures that gradually gravitated away from their mother countries.
At the end of its reign, the Stromic monarchy gradually lost control of its conquered domain. While some, namely Gilneas, remained loyal for an extended amount of time, others (Such as Lordaeron) were eager to declare independence. This is likely because of cultural differences -- the Lordaeronian language is far more different from Common than Gilnean, meaning the Imperial identity had left by the time modern Lordaeronian was finished.
Many wars, alliances, and most everything in-between occured between then and the Second War. Simply put, this is because the Orcs were just too big of a threat -- and so began the first Alliance (of Lordaeron), which would be dissolved after the Third War, culminating with the Fall of Lordaeron. In modern times, many Humans live in Stormwind and minor Kingdoms, while a good deal of main Kingdoms are in disrepair or under enemy occupation.
Almost all major Human nations have their own language, most of which stemmed from Vrykul and the "Universal language" of Old Common. Further, these nations are bilingual with the extremely easy Common being taught alongside that of their own area. Major countries are not the only ones with their own tongue, however. Areas such as Ardeiria and Havre have either evolved or created their own, which have managed to reach the same level as advancement as those such as Lordaeronian or Stromic. Because of this, the remaining main Kingdoms do not only have their own and Common, but minor ones held by minority groups therein. Below are the languages and their real-life proxies:
Kingdom of Gilneas: Gilnean -- Cockney English
Kingdom of Kul Tiras: Tirassi -- Spanish
The Vrykul: Vrykul -- Icelandic
Old Stromic/Common -- Macedonian/Latin
Kingdom of Stromgarde/Arathor: Common/New Stromic -- English
Kingdom of Stormwind: Common -- Only Common varying dialects, as well as bastardized Old Stromic in certain places.
Kingdom of Alterac: Alteic -- German/Romansh
The Wickers: Galui -- Irish
Magocracy of Dalaran -- Common only, many immigrants.
Kingdom of Lordaeron -- German
Common is the product of Old Stromic's modernization, incorporating elements of both Vrykul and Old Common and, inadvertently, making their language far more approachable. The basics of New Stromic were made before the Empire's split, and so it had spread enough to remain a "Universal language", while Old Stromic is only present in a handful of scattered areas and Old Common is essentially dead.
Efforts by past Emperors to unite humanity under one tongue has resulted in most, if not all, regions of Humanity speaking Common. In some regions, such as Kul Tiras' fringe islands that are closer to Stromic territory, smaller areas have completely lost their nation's original tongue due to these efforts.