Tolkien loved the languages. Since he was a child, he loved the sounds of different languages and very early he began to invent his own languages. For example, he started to invent the language of the Elves when he was only twelve years old. Not less than 15 languages appear in Tolkien's books (English not counted.) He invented these languages in a so detailed way that it is hard to know how many words of his languages exist. Estimates from the exisiting elvish words go from 10.000 to 60.000 words !

:: Westron ::

Westron was also called the Common Speech and it was the language of the West-lands of Middle-Earth in the Third Age. In the course of that Age, it had become the language of nearly all the speaking people except the Elves. Even those people who shunned other people were able to speak it, although they often retained a speech of their own. For example the Rohirrim, the people who lived in the plains of Rohan, spoke their own language but as a second language they spoke Westron.

In Tolkien's books, Westron is translated into english (respectively german or french.)

:: The languages of the Elves ::

The Elves of the elder days became divided into two main branches : The West-Elves (The Eldar) and the East-Elves.

Of the Eldarin languages, the two most important ones were the High-Elven, Quenya, and the Grey-Elven, Sindarin.

Quenya was an ancient language of Eldamar beyond the Sea. It was the first language which was recorded in writing. But soon it was no longer a spoken language but became an "Elven latin" which was only used for ceremonies by the High-Elven.

Sindarin was in origin akin to Quenya. It was the language of those Eldar who, coming to the shores of Middle-Earth, had not passed over the Sea but had stayed in Beleriand. With the changefulness of mortal lands also their language changed and became far estranged from the Quenya.

The High-Elves, who lived among the Grey-Elves, adopted the Sindarin for daily use.

To write down their language the Elves use the Tengwar. See picture of Tengwar (3rd age of Gondor) below.

:: The Language of the Dwarves ::

The Dwarves called their secret language Khuzdul. Only a few people were able to learn this language as the Dwarves kept it hidden. Only a few words are known, mostly the names of towns and mountains. For example Moria is called Khazad-Dûm in the language of the Dwarves.

The Khuzdul almost didn't change during the years.

To write down their own language and also the language of the Elves the dwarves didn't use the Tengwar. They used the Cirth (The Runes). A special form of the Runes were the Long-Runes, also called Angerthas Daeron. The Dwarves modified these LongRunes and developed the Angerthas-Moria.

:: The language of Men ::

The Westron was a Mannish speech, though enriched and softened under Elvish influence. It was in origin the language of those whom the eldar called the Atani or Edain, "Fathers of Men", being especially the people of the Three Houses of the Elf-friends, who came into Beleriand in the First Age and who aided the Eldar in the War of the Great Jewels against the Dark Power of the North. After the overthrow of the Dark Power in which Beleriand was for the most part drowned or broken, it was granted as a reward to the Elf-friends that they also might pass west over the Sea. But since the Undying Realm was forbidden to them, a great isle was set apart for them, most westerly of all mortal lands. The name of this Isle was Númenor (Westernesse.) The elf-friend who started to live there were called Númenoreans, the Kings of Men, whom the Elves called the Dúnedain.

The Dúnedain were able to speak the elvish languages, for their forefathers had learned Sindarin. Their men of wisdom also learned Quenya. But their native speech remained their ancestral Mannish tongue, the Adûnaic, and to this in the later days of their pride their kings returned, abandoning the Elven-speech, save only those few that held still to their ancient friendship with the Eldar. In all the forts and havens of the Dúnedain spoke Adûnaic and mixed with many words of the languages of lesser men, it became the Common Speech (Westron.)

From : Appendix F, Lord of the Rings

:: The Language of the Hobbits :: 

The Hobbits spoke the Common Speech (Westron) and there is no record of any language peculiar to Hobbits. They have always used the language of the people among or near whom they lived.

Thus they quickly adopted the Common Speech after they entered Eriador.