How about giving cabals the ability to have secret languages they can use so when you have wars or group fights going on there can be more outside speech with each other vs cabal speech. May increase visible RP a bit to see the enemy yelling to each other in a language you can't understand.
If you've mastered the language of a race and are in a city owned by the race you could get more discounts from stores or even get hidden quests due to being so familiar with them.
If you don't use a language for so long you begin to forget it, after mastering a language it is quicker to relearn it though.
Choose your starting language at creation, sure you're a Dwarf but maybe you were abandoned and raised by Elves.
Would it be possible to add a slang or modified form of speech for each city in common? These cities are far apart and people wouldn't speak exactly the same across a continent.