I thought she was assumed Canadian thanks to the Pogos = Corndog thing from her as well as the french fluency.
A lot of people do say that she's in Canada because of the pogos line.
My own personal belief is that she's in the US, as most of the evidence we have points towards that. The most obvious bit being that Katrina Ange straight-up asked Rosemi where she lives, and she replied with "America." She
could have meant Canada, since Canada is in North America, but I don't know of any Canadians who reply to "where do you live?" by saying America based on that technicality.
Her Nintendo account uses an American flag. Additionally, back when she did her Rosemi game show, she created a series of polls for the stream using a third party site. That site mentions the location of the person who created the polls, and stated that they were created by a user in the United States. It's also worth mentioning that in her PL as Miki she described herself as being of Japanese and
French - not Canadian/Quebecois - parentage.
So basically, Rosemi is either in the US, or she is in Canada and is purposely hiding that fact by changing her Nintendo account location, using a VPN when making polls, being vague towards Ange, etc. Either is perfectly possible, but given how the confirmed Canadians in NijiEN - Elira, Millie, Enna, Nina - have no trouble admitting that they are from Canada, I suspect that Rosemi isn't in Canada at the moment. My
guess is that she was perhaps brought up in Canada, at least partially, but now lives in the US. She has mentioned that her family has moved around a lot for work - she even spent a bit of time in England in the past - so this isn't a hard thing to imagine.