A few questions about languages

[KaiBlob1]

In the process of making this thing, I have come across a few questions about languages.

1. Is Ysoki a common or uncommon language? AoN lists it as common, but this seems odd as all other non-core ancestry languages (tengu, amurrun, iruxi, etc) are listed as uncommon in bestiary 1, where they appear for the first time (Tengu is actually not listed in the language table for Bestiary 1, despite appearing for the first time in a monster in that book).

2. How would one learn ancient osiriani, for the purposes of Living Monolith Dedication? is it an uncommon language one can just learn through multilingual, or is it closer to a secret language, seeing as it's probably not spoken at all anymore?

3. Is there a reason Leshies can't learn arboreal? I get why their "native" language is sylvan, but it seems like arboreal should at least be on the list of languages they can learn.

Thanks in advance for any answers

[Deadmanwalking]

For #2, Ancient Osiriani is a dead language but pretty well known among scholars in-universe, so it's almost certainly at least Uncommon, but assuming it's rarer than that is deeply weird and makes the setting no longer make sense.

So it's definitely Uncommon.