The Westron or Common Speech is the closest thing to a universal Language, at least at the time during which The Lord of the Rings is set. Westron is an English word, derived from West, not a word from the language itself. Westron is a translation of the original name Adûni, and "Common Speech" translates the Westron term Sôval Phârë, of identical meaning. In Sindarin the language was called Annúnaid (Westron), or Falathren (Shore-language).
The Westron speech is derived from the Adûnaic tongue of Númenor, and originated as a creole language on the western coastlands of the continent of Middle-earth, when the Númenorians established trade outposts and forts there. From there, it spread to most of the lands through which the action of The Hobbit and The Lord of the Rings passes, with the notable exception of Mordor.
According to Tolkien's fiction, Westron was the language spoken and understod by the protaginists of The Hobbit and The Lord of the Rings. Many names of characters and places, in the book's "reality", were in Westron.
However Tolkien, mentions that Westron was presented as having been completely replaced by English in the text. This had certain important implications: first of all, proper names with derivations understandable or evident to speakers of Westron had been translated, to preserve the effect to the English reader. Thus, names like Baggins, Bagshot Row, Peregrin, Rivendell et cetera, are not the actual names as spoken by the characters but are presented as translations.
Outside the context of the story, it is clear that most of the "original" forms in Westron or other languages were devised by Tolkien long after the English "translations" were chosen.
Rivendell ("cloven valley") was actually called Karningul, and Bag End was actually called Labin-nec, after Labingi, the real form of Baggins. In some cases the explanations became quite involved, such as the river Brandywine (Sindarin Baranduin, "golden-brown river") was actually called Branda-nîn, a punning Westron name meaning "border-water", which was later punned again as Bralda-hîm meaning "heady ale".
This logic went one step further by also presenting all languages akin to Westron in languages related to English, so that their "understandability" by the protagonists be simulated to the English reader: Rohirric, the language of the Rohirrim was translated by Anglo-Saxon, as Rohirric is an archaic relative of Westron, much as Anglo-Saxon is an archaic relative of English; Dalish, the tongue of Dale (from which came the names of the Dwarves of Durin's Folk), was translated into Old Norse, a language related to Anglo-Saxon and modern English as Dalish was related to Rohirric and Westron; the names of the Stoors as well as some Breeland names are Celtic.
This utter replacement of Westron by English was taken so far that some sources that should give actual Westron have been turned to English too. For instance, in Moria, an illustration of the runic text on Balin's gravestone is given. The text is said to be written in both Khuzdûl and Westron. But while the first part of the inscription seems to really be a bit of Khuzdûl, the second part is actually plain English, just written in cirth.
The corpus of Westron is small; the majority of the words come from Appendix F and the creation of it in The Peoples of Middle-earth. Additional information was published in Tyalië Tyelellié 17, in 2001. Even now, the corpus is very small.