As I've re-read my last post, it occurs to me that while it's a benefit to encourage some level of richness to a setting, it's also potentially a trap for the unwary. Let me explain.
The example I gave in the last post is how a language extremely closely related to Ma'anyan and other eastern Bornean languages ended up on Madagascar, and in fact the native language of the whole of Madagascar and Mayotte, where it outnumbers all of the rest of the speakers of the entire east Barito language branch put together. The entire Austronesian diaspora, and particularly that of the group that moved to Madagascar is a somewhat mysterious process; the how and why of it is unknown, and all kinds of interesting stories regarding the move could be told, albeit speculatively.
While these kinds of mysteries are part and parcel of the real world, and therefore encourage richness and verisimilitude in a fantasy world, they are also potential distractions from the efforts of a gamer or a writer, unless catering to a real fan of simulationism or exploration. I think most fantasy fans have at least some element of that; they like their settings to feel "real." Even as they understand that of course they are not, they are likely to demand sufficient detail to maintain an illusion of reality to aid in suspension of disbelief. Also, I think that most fantasy fans like exploring settings, and seeing what the gamemaster or writer has come up. Tolkien's richness to his Middle-earth setting is frequently cited as one of the main draws of the setting and story both.
But, more is not necessarily better, and even Tolkien knew enough to put most of his detail in other sources, out of the way of the main story. The fact that most fantasy fans are at least somewhat fans of exploration doesn't mean that they want to experience vicarious travelogues or ethnologues of fiction places and peoples. I've seen several games (and novels!) fall victim to this trap, and the end result is that the characters and the stories that surround them suffer as a result.
Be very careful of integrating details into your campaign setting. For the most part, if there's a mystery or secret, the point of it is so that the characters can discover the solution to it. Occasional red herrings can be fun, but a world full of mysteries and secrets that never get solved might be realistic... but ultimately probably isn't very fun to explore after a while.