Well, it appears natural for the relation between Maria and Nagi to be familiar, as they almost grew up together, and Maria is for Nagi a Mother/Big Sister substitute.
Another of my rants on the French edition is that they make Hayate and Maria use a too familiar speech when addressing each other (resulting in weirdly dissonant discourse)...
... when it's clear that their junior-senior interactions are much more formal (and that there
are ways of rendering that in French
).
Oh well... perhaps I'm a an old-fashioned relic, as I am used to formal speech and addressing peoples by their surname, and they adapted it to the more familiar and modern ways of speech of nowadays youngsters.