When a player gains ranks in Linguistics, they learn a language. What exactly happens when a headband of vast intelligence grants ranks in linguistics? Does the wearer gain new languages, just as if they had taken those ranks at leveling up? Or do they only gain the other benefits of the linguistics skill?
I've been scratching my head at this one with a player for quite a while, and for how many problematic cases and interpretive issues we've been able to come up with, I'm shocked that I can barely find any information on this online. The issue is that linguistics is the only skill that has a "side effect": gaining a rank grants a static ability that has nothing to do with checks related to that skill.
Here is the relevant rules text for a headband of vast intelligence:
The headband grants the wearer an enhancement bonus to Intelligence of +2, +4, or +6. Treat this as a temporary ability bonus for the first 24 hours the headband is worn. A headband of vast intelligence has one skill associated with it per +2 bonus it grants. After being worn for 24 hours, the headband grants a number of skill ranks in those skills equal to the wearer's total Hit Dice. These ranks do not stack with the ranks a creature already possesses. These skills are chosen when the headband is created.
We also have this related FAQ that treats the topic of a Headband of Vast Intelligence and bonus languages. (For clarity's sake: bonus languages are based on your intelligence modifier, regardless of ranks in the linguistics skill. Languages gained from linguistics are not "bonus" languages, at least as far as this question is concerned.)
Intelligence: If my Intelligence modifier increases, can I select another bonus language?
Yes. For example, if your Int is 13 and you reach level 4 and apply your ability score increase to Int, this increases your Int bonus from +1 to +2, which grants you another bonus language.
Technically, Int-enhancing items such as a headband of vast intelligence should grant a specific language (in the same way they do for skill ranks).
Finally, we have this rules text from the Linguistics skill:
Whenever you put a rank into this skill, you learn to speak and read a new language.
So there's a number of questions here:
If a character crafts a headband with linguistics as a chosen skill, do they learn a language for each of those ranks? The headband "grants" a number of skill ranks. Does "granting" these skill ranks to a character count as a character "put[ting]" a rank into linguistics?
If so, how are those languages selected? Are they "fixed" in the headband? Or are they chosen when the character first attunes to the headband?
If the character gets to choose the languages, do they get to choose different languages after taking the headband off and putting it back on, say, a week later? If not, is there a rules-as-written justification for why not?
If the character gets to choose the languages, are those choices fixed for future wearers of the same headband, or not?
If the languages are "fixed" in the headband, when are those selections made? At creation time? Does the crafter choose a "pool" of 20 languages to choose from, or do they define the languages to be made in a particular order?
What happens when a Tengu, or some other creature with the Gifted Linguist racial trait (which grants two languages per rank in linguistics) dons the headband?
If the character gets to choose the languages, but is limited to a single choice, what happens if they later lose the headband and take a rank of linguistics on their own? Do they need to make the same choices (and in the same order) as they did when they had the headband?
If the character already has ranks in linguistics (say, for example, a fifth-level character with one rank), and then attunes to a headband of Vast Intelligence, do the headband's languages override their previous choices? Do the previous choices "stack" with the new ranks? (The skill ranks themselves don't stack, of course, but learning a language seems to be a side effect.)
If the languages are fixed at creation time, does the creator need to know those languages?
Rules-as-written answers are preferred, but if there is an existing body of rules-as-interpreted evidence that would suggest solutions here, I am open to them.