I want to be able to talk to whomever we may come across. Despite keeping Linguistics maxed, I don't know every language out there; enter Tongues.
There are several items which grant Comprehend Languages (useful in its own right), but I have not found one which guarantees that I can respond in a language/way that the other guy can understand. This is my goal; Tongues is simply the most obvious (to me) way to get there.
I'm simply looking for communication; I'm already plenty convincing, and I may want to talk to the King without having to break into his mind first.
I'm a 13th level Cleric (Protection and Healing domains) with a fair bit of gold (roughly 30k GP) available. The party's other caster is a Bard.
Finding a friendly Wizard to cast Permanency into a Ring of Spell Storing is, of course, a possibility (other party members might well chip in for the ring, since I'd be able to keep it full of useful spells most of the time), but it's subject to being Dispelled relatively easily (which requires finding another friendly wizard).
The GM is open to custom magic items, and is the fall-back solution.
Best Answer
You have a lower level spell that will do this Voluminous vocabulary is a third level cleric spell/second level bard spell from ultimate intrigue. Now it only lets you you speak one new language per casting* but that can be ANY language aside from secret ones(such as druidic) So with one casting you will be able to speak the king's language for 8 hours. You do have to know of the language you want to speak, but with many points in linguistics just having heard of a language should be a fairly low bar.
*if you already have lots of points in linguistics you can gain multiple languages per casting.
Since it is third level (2nd for a bard) you can also brew potions or craft wands of it yourself fairly cheap, you could even give some to the king as a gift, so he can speak common when he needs to.