I'm pretty sure that my theory about it accelerating memory degredation rather than erasing memories is correct, since one of the game's themes is growing up and moving past your childhood, which is basically spelt out with Ultimecia's final words which weren't fucking translated right aaaargh
I feel like it's one of those things where in-universe there is not enough research to make any concrete claims, and out-of-universe the writers simply didn't bother thinking about it as much as we are right now.
The question I have is Quistis. From a direct "GF usage causes memory loss" context, Quistis has clearly been using GFs longer than anyone else in the party, since she graduated and became an active SeeD and instructor long before the other SeeDs. But her memory doesn't seem to be noticeably more missing or degraded, and she remembers the orphanage once prompted by Irvine, just like the others.
And from a "GF memory loss is part of the theme about growing up" context, Quistis
also should have forgotten more than she does, because according to Quistis, back in the orphanage she looked up to Matron Edea, and wanted to be all mature and grown-up like her. (According to the Japanese script, anyway.) Which explains her "big sister" attitude towards the others, but which should also mean she would have left behind all those childhood memories.
Now, it's entirely possible memory degradation via GFs happens at different rates for different people, or there might be some other reasoning, but that's just us coming up with justifications, rather than anything confirmed by the text. Hence my comment about us thinking about this more closely than the writers likely did.
Amusingly, Vanille doesn't have an accent in the native Japanese audio track, much like how Selphie doesn't have an accent in the English text.
It's kind of fascinating, because Selphie only speaks in Kansai dialect maybe
twice: once when she's in despair after seeing the missiles fired at Trabia Garden, and the next potentially when she's talking with the Trabia Garden survivors. (I don't know if she does it in the option Omicron picked; she does do it in the option Omicron doesn't.) Every other time, Selphie speaks Standard Japanese, as filtered through her teenage-ness.
Meanwhile, Vanille in English speaks Australian the entire time, so it's a core part of her characterization. And unlike how it's relatively simple to switch between Kansai and Standard/Tokyo Japanese, it would probably sound weird if Vanille could switch her accent on and off at will.
Part of it might be FFVIII being unvoiced. Kansai dialect is partly in the word choice for the grammar, and partly the slightly different intonations and emphasis on syllables. With just plain text, we can only rely on the first part.