I'm french and i know the answer
In english speaking country you're learning that sqrt(-1) = i. This should be seen more as second way to express and write i than an actual operation
Thus you're not allowed to expand basics sqrt rules with negatives numbers
For instance, you can't write sqrt(-1)*sqrt(-1) = sqrt (-1*-1) (and then i*i = sqrt(1) = 1)
In france, we just learn that the sqrt(-1) is a nonsence although i is well defined by i² = -1. So we don't encounter paradox like that.
Sorry for my possible bad english
