Westron

The Westron speech is derived ultimately from the Adûnaic tongue of Númenor, and originated on the western coastlands of the continent of Middle-earth. 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.

Being the language spoken by all of the characters, Westron is represented in The Hobbit and The Lord of the Rings by English. This has certain important implications. First of all, proper names with derivations somewhat evident to speakers of Westron have been translated, to preserve the effect. Thus, names like Baggins, Bagshot Row, Peregrin, Rivendell etc., are not the actual names. (For example, Meriadoc Brandybuck's actual first name is supposed to have been 'Kalimac'. 'Meriadoc' is designed to maintain the reference to merriness contained in the original name Kalimac.) For similar purposes, the language of the Rohirrim[?], related to Westron, is represented by Old English.

I know - someone really needs to rewrite that last paragraph