Social Trait: Adopted

[Azketh]

This is not the question you think it is.
This trait does not give you access to racial traits of other races but to Traits that are in the Race category within Traits. The same place that you got the trait Adopted from, just not a Social Trait but a Race Trait. Now, with that out of the way.

Adopted has the following text:
You were adopted and raised by someone not of your actual race, and raised in a society not your own. As a result, you picked up a race trait from your adoptive parents and society, and may immediately select a race trait from your adoptive parents' race.

So you are not of this race, but you were raised within it. It doesn't suggest how long, but if it's long enough to have some societal trait as part of your character why is there also not an option for the specific language of that race/ethnicity and only that language?
So if your adopted parents were Elves you get access to elvish, this is not to say you have it automatically, but that you could take it. You should even have the option to loose your own racial language and use that the gain the adopted racial language.
If this actually exists could someone please point me towards this.

[Belafon]

There are a few that are race specific like Child of Two Peoples (for humans and elves).

Speech of the Wilds gives you any language, but you do have to be adopted by Aasimars.

As always, ask your GM. I think it's perfectly reasonable to have "knows the racial language" as your race trait. Not the least bit overpowered and I would definitely let a player who asked do so.

Alternatively, you can just put a point into Linguistics at first level and choose the racial language.

As for why:

Honestly it's probably an oversight.

I'm trying to think of a real-life situation where it might make sense but I'm drawing a blank. I know a couple who (if you go back far enough) are ethnically Chinese, adopted a Chinese baby, and spoke Chinese to the child in the home. But the kid also speaks English just as well as the parents do (English is the parents' first language).

You're right. Even if the child is older, they are going to pick up the language if they can pick up the trait.

[Azothath]

Adopted (Basic) trait

[Mysterious Stranger]

Not all adoptions involve raising the child from infancy. If the child was adopted as a teenager, they might not pick up the language of their adopted parent.

[Java Man]

In my games every PC (that doesn't have the xenophobic language feature) starts w common and their racial/ regional/ ethnic language. I'd allow (maybe require?) a character with the adopted trait to swap that language. But that's house rule.