When I read the game dialogue, I am reading it in English. So the voice I hear in my head is also speaking English. They don't have different "accents" to me but they do have different vocal affectations: Marisa is a bit gruff and tomboyish, Reimu speaks distinctly, Yuyuko has this kind of light-hearted flighty tone, and so forth.
This, pretty much. I read them with sorta accentless english with variations mostly coming from the tone, inflection, pitch, and speech style (informal, gruff, refined, etc).
Marisa naturally uses an energetic, gruff, tomboyish style, with a lower-than-average pitch (that part comes from the tendency for her to be voiced as such in fan made stuff like fantasy kaleidoscope), informal and casually disrespectful, youthful rather than girlish;
Yukari uses that refined japanese noblewoman style of speech that endeavours to reveal nothing about what they actually think or feel, affected, obvious fake surprise/confusion, that sorta thing. The kind that makes communication sound like a verbal spar instead of a conversation. When she's in a lighter mood, its the same thing but with a more affectionate and/or teasing tone. Yuyuko is similar to Yukari with added mirth, and so forth.
The only one I can think of that I wouldn't assume is just the typical reading would probably be Orin, who reads as using "cliche delinquent Japanese" for some reason.
If I had to assign actual accents to them, the mentioned four work with "anime japanese" since those speech styles are pretty prevalent in that medium.
I think I'll just give everyone accents next time I play through one of the games. Yukari as upper class British, Marisa as some American casually in-your-face accent, Alice as either upper class British or Northern Irish (that one with the soft lilt) except for in IN, where it'd be more of a valley girl thing, and gaelic for the Oni, why not.
I'd probably say French for the vampire duo, though given their spell cards a norse-derived accent would be neato. Oh, and Patchouli using lazy Australian (start with a mild english accent, then kinda push the words out instead of properly enunciating, drop any unnecessary phonemes, make up words or phrases to fill the gap when you can't immediately think of the proper one, and expect the other party to figure it out from context. If they can't keep up, eh, their loss) shall verily be a thing.
Also Russian Eirin because Yagokoro.
