Some RPGs have multiple languages, and player characters may not all speak the same set of languages. If you're speaking in a language everybody knows (such as "Common" or similar), you can obviously type your message normally. However, if you want to send a message to everyone that speaks Elven, for example, it would be tedious to ask who speaks Elven, and whisper to each one individually.

Instead, you can try this:

• Create a character sheet named "Elven". (Or whichever language you want to work with.)
• Add each player with a character who knows Elven to the list of players that can edit Elven.
• When speaking in Elven, whisper to the Elven "character," and the whisper will be received by all of the players with an Elven-speaking character. (ex: /w Elven Watch out for that dwarf. He knows too much!)

With a "character" for each language, each controlled by all of the players with a character that speaks the given language, you can thus have entire conversations hidden from the players that can't understand your speech!

Note: While this solution will exclude players from the conversation that should be excluded in-character, the players themselves may feel excluded from the game. If the owner of the game is a Pro subscriber, he or she could use the Roll20 API to, for example, send a message to the chat about how the other characters are speaking in an unknown language. See API:Chat for more information about what the API can do with chat messages.