>>53371438She says she's Japanese, and she accidentally showed full computer screen a few times - the system language and time were set to JP, and there were a few JP-specific apps installed. So she either tells the truth, or she's extremely committed to her keyfabe.
Her voice is plain weird. I've seen people place her accent as "Russian" a few times before - and it wasn't just the local rrat enthusiasts. It always sticks out to me because I speak Russian myself - and she doesn't really fit the usual "Russian accent" patterns.
There are certain consonants and pronunciation quirks that easily bleed over from Russian to English, and are hell to exorcise - and she just doesn't have them. Nor did she ever show any ability to read or speak Russian on stream, as far as I'm aware. Same goes for Ukrainian, which has a considerable overlap with Russian - but I can't vouch for other Slavic languages.
I'm not sure why people think it's Russian accent specifically. Maybe because her voice has that "exotic, rough" vibe to it, and that vibe often comes from a Russian accent. It might resemble German to others then - German accent being another source of the same vibe.