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.
In this case it makes more sense to modify the effect than it does to try and stack multiple copies of itself. Boots of Speed are a CL10 item and they offer ten rounds of haste per day because Haste cast at CL10 has a duration of ten rounds. Make them a CL20 item, and that'd translate to 20 rounds of haste instead.
When it comes to the price, the working for the original item is:
Spell Level (3)
* Caster Level (10)
* 2,000gp (use-activated effect)
/ 5 ("1" charge per day)
= 12,000gp
If you make that CL20 instead, you increase the base cost to 24,000gp, or 12k to craft. You could achieve the same effect for the same cost by calling it two charges per day for a total of 20 rounds, instead - but making the item CL20 instead increases its resistance to magical interference by others.
I am guessing your character is not a 20th level caster. Luckily, as per the sidebar on the SRD, no part of the magic item crafting rules actually requires the crafter to meet an item's CL; you can create your item at whatever CL you like, as long as it's high enough to cast the required spells and you can afford the cost.
Best Answer
You can wear them, but their effects might not stack.
In this case, two rings of protection won't stack, because the rings provide a deflection bonus to AC, but deflection bonuses don't stack with other deflection bonuses.