One PayPal account can handle multiple currencies, however, I have seen some stores use two separate accounts where one is registered in the US and one is registered in Canada for tax reasons. I'm not a lawyer or accountant, though, so you might want to check with one to see what the best approach would be for your business as far as one PayPal account or two.
Now, on the technical level - what PayPal account is used to accept payments in Magento is set at a website level. So unless your French and English stores are setup as separate websites in Magento (and not just separate store views, which is the more typical approach), you're going to have to use the same PayPal account.
If you're open to changing how your site is setup, then you could create a separate website for the US customers and US currency with one Paypal account and another site for the Canadian customers and Canadian currency - that is definitely possible in Magento. You could even still offer both French and English on the Canadian website.
--Joshua Warren - @JoshuaSWarren on Twitter
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