The basic language should always remain English. However, there should be some sort of way to communicate with tribe members without having friends and foes knowing what you're actually saying. There are many reasons for this...
Some information should only be heard by your tribe members. Calling someone a traitor without their knowledge, gives you and your tribe members the opportunity to react before the player does. In addition, with one word you could make your tribe flee before the enemy could react.
Spying would be a new ability which would allow you to learn a player's slang language. This can be done by talking to them or by sneaking up on their conversations. Training this skill would also allow you to learn other languages faster. It would be a great way to ambush players and have an idea of what they're up to and where they're going. It could also allow you to react faster than they do.
Basically I'm suggesting players to be able to formulate a language, but not entirely. Maybe players are restricted to redefine a certain number of words. This would make tribes think critically of what word they should redefine to benefit them the most. Whether it's for ambushing or for security purposes. I'm not saying that slang has to be "ghetto".
Of course we could just make up words and tell players what they mean in our tribe, but it would be nice to have a built in system for it. This way it could be more organized with little to no memorization require. Players should also be given the opportunity to name their language. Languages can also be unlearned. Ex-tribe members are allowed to keep the language and can use it for revenge.
Let's say I set the word "traitor" to "trip". Whenever I say traitor, my tribe members would see the word traitor while everyone else who's not familiar with the language would see trip.