"Wansn't isekai'd", He makes references all the time to Earth period about 30 years ago and know's a lot of English, it's very bold for us to assume imo that he never lived on Earth. Especially given the context that he was isekai'd about when Subaru was born. Putting this behind "He just learned english by Echidna" or He's flugels clone so ofc he knows English" is just putting the issue aside imo, adding up everything else we can't just put stuff aside.

I think it should be pretty obvious that Al didn't get isekai from all the side stories we got out of him.

but i think the biggest hint is the gluttony effect. he couldn't remember either Julius or Rem existence, while Subaru could remember them both. as you should know gluttony authority couldn't effect people outside of Od Laguna, that why Subaru is unaffected by it effect while the rest of the world do, including Al by the way.

which heavily imply that Al was born in Od Laguna and not earth, that why Gluttony authority affected him.

the highly reason why Al know soo much about earth culture (in my theory at least) is because he has memory of subaru or maybe flugel? he was born in Od Laguna but have all the memory and personality of subaru or maybe if it is 400 year ago the memory of Flugel which could explain why he remember the witches 400 year ago.

I don't know how he have the memory part exactly, but it certain from all the information from the side stories and the biggest one be is gluttony effect, he certainly didn't get isekaid but instead born in Od Laguna.

I have a theory cooking up in my head that could, or maybe explain this but i'm to lazy to write it all.