RJ on how he invented the Old Tongue:

"The words come partly from Gaelic, Russian, Arabic, Chinese, Japanese. The grammar and syntax I believe I invented myself, although it's possible that another language uses the same. Of course, just as with English, I have deliberately put in some very illogical inconsistencies." [America Online chat, 27-6-96]

Why doesn't the Old Tongue bear any relation to the New Tongue spoken by Rand and Company?

It probably does, but RJ has kindly "translated" the New Tongue into English so that we can read the books and he can make lots of money :). Old Tongue phrases are not "translated" in order to add "flavor" to the story. Think of Tolkien, who did create entire languages. Even he didn't write The Lord of the Rings in Elvish or some other Middle-Earth language.

Is there an Old Tongue Dictionary?

There are several on the Web. The first, and one of the most complete (which many unethical people have plagiarised) is The Compleat Old Tongue, compiled by Aaron Bergman, Andrea Leistra, Don Harlow, "Mark," and "BAClubb." It can be found at http://linuxmafia.com/waygate/old.tongue.guide.

Why do all the characters speak the same language?

At the East of the Sun con, held in Sweden 16-18 June, 1995, RJ explained the concept of the unified language. He said there had been a single language in use all over the world (the Old Tongue), and the writing and printing of books continued throughout the Breaking, albeit to a very limited extent. The written word introduced a very large conserving factor in the language-change mechanism. (report by Karl-Johan Norén)

Chad Orzel gives a further explanation of why we shouldn't expect a lot of language drift:

1. Point the first: three thousand years ago, we have the AOL. At this point, it's pretty safe to assume that everybody speaks the same language.
2. Then, we have the Breaking. People scatter, keeping what little knowledge they can intact. Chaos reigns. Whatever is settled upon at the end of the Breaking will henceforth be called "The Old Tongue." Presumably, the Old Tongue is spoken primarily on the main continent. We have no idea what happens in other areas of the globe (Seanchan and Shara), but this doesn't really matter, as we've never heard a Sharan talk, and the Seanchan are the product of an invasion from Randland proper.
3. Languages drift for a while, but it doesn't matter how much, as Artur Hawkwing pops up, and conquers everybody. A new lingua franca is established, which shall be called the New Tongue, or "English" as Jordan has translated the whole thing anyway. Presumably whatever they speak has clear roots tracing it to the Old Tongue. Hawkwing conquers the entire main continent, some of Shara, has dealings with the Aiel, and his descendents conquer (and become) Seanchan. Again, everybody speaks the same language, with a few quirks here and there.
4. Almost a thousand years pass, and languages drift. Now, just how far can we reasonably expect them to drift? For convenience, let's look at modern Europe, or at least those parts of Europe which formed the core of the Roman Empire. This gives Spanish, French, and Italian, plus a handful of others which I'll ignore completely. It's been fifteen hundred years or so since Rome fell, how different are these three? And what are the sources of the differences between them and Latin? Not that much. If one knows Italian, one can puzzle out a lot of Spanish, if the Spanish-speaker talks slowly, and vice versa. All four languages have many similar words. Now, consider what we've had happening in Europe in those fifteen hundred years: Most of Europe was at one time or another overrun by those pesky Germanic types, and Spain got invaded by Moors. We've had Russian-type languages pushed in from the East, Scandinavian from the North, and I-don't-know-what from the South.

   Now, look at Randland. Who's going to invade? Hawkwing basically conquered the world, so there's no one who can bring another language in from outside. And even if there were such an incursion, the language being brought in would still be pretty close to the Old Tongue, since everybody spoke the same language back in the AoL. And what do we have? We have a good number of funny accents, the Seanchan slur everything, to the degree that Our Heroes have trouble understanding them, the Aiel have a number of odd words for things not found in the Wetlands, Bayle Domon do be using odd verb forms, and the Taraboners, they put the words in the wrong order, yes? Is this really that unrealistic? Given the utter lack of invasion from outside, or even the possibility of same, I don't find it hard to swallow the relative uniformity of language in Randland.