Ok, for most races that's obvious. Elves learn elven because they grew up in an elven country or something.
But there are other races where it makes not too much sense. Most of the Native Outsider races for example.
The way I understand it, Aasimar and Tieflings, and also the new "half elemental" races like Ifrits, are born to normal human parents, but are a throwback to their ancient heritage, and not usually a seperate culture and race (they have no homeland, usually not born within the race, etc)
How do those learn celestial or infernal or ignan? Does it just come in a dream to them? Is it assumed they join a "tiefling commune" where they learn it?
Magic.
Genetics
racial memory/genetics
The language is in the blood!
hmmm, makes me think about vamps in a different light. Suck blood, get some racial knowledge or language...
Same thing for half-elves and half-orcs raised among humans. If it bothers you then remove it from your list of known languages.
It could also be that since most aasimar and tieflings (for example) are all too aware that they are different, they go around trying to learn as much as they can about what they are beginning to suspect about their own heritage.
Its sort of the age-old, "Who is my father?" search.
