There are many things in Smash that aren't very true to their characters though. When has Luigi ever rocketed himself across the screen? Luigi's final smash trophy says it comes from the darkness in his heart from being second to Mario all the time, but Luigi has only shown love and admiration for his brother, never envy. The star fox characters weren't given jet packs and guns like they were seen with in the GameCube Star Fox games, instead he kept to the largely random move sets they were given before. Why do the mother characters shout PK instead of PSI? I'm not familiar with the Mother franchise, but is PK starstorm even an attack in that series? Rest is clearly designed as a joke move. For many characters Sakurai does keep to that character's history, it would be odd to have Mario using Link's arrows or something, but he just as often makes things up just to fit smash but could have had something from the character's past instead. Sakurai seems to use tradition when it pleases him, but bucks as soon a he gets an idea he feels is better or if tradition starts o not mesh with his vision of what smash should be like.
The only reason ever given for keeping Marth speaking Japanese was that he only had games released in Japan and never anywhere else. Shadow Dragon was released in the U.S. after Brawl. What other reason is there that was ever given? I did say that my thing with understanding Marth's character was more a personal issue than a design issue, but it is worth noting that some people feel closer to characters who speak the same language as them than ones who do not. If there is a big enough group of people who react positively to an English speaking Marth, than he should be given an English voice. We don't have any polls on the matter so we can't really say what would be better received, but it is worth keeping in mind.