I don't think they will dubb Marth in English. Mainly because his heritage is true to Japanese, that's where he is most recognized, and whether the franchise spread here or not, it will most likely stay his trademark seeing as it solidified Fire Emblem's spread to NA. Would be like giving G&W actual words to say, or like giving Banjo-Kazooie actual voices after all these years when it's the true nostalgic feeling that keeps people coming back. Nintendo could have even potentially dumped Mario long ago if he hadnt coined them the catch phrase "Here we go!" And made it a household reference point, even now.
My point is, Nintendo is gonna stick with what draws people in and what makes money. Bottom line, if it ain't broke, don't fix it.
(P.S., I still say Banjo-Kazooie should make it into the next Smash Bros... Gruntilda too.)