We know that you have to say the name of your destination clearly and distinctly when you're using the Floo Network. For example, Harry says "diagonally" instead of "Diagon Alley" and gets misdirected. This comment on an answer to How did Nagini enter the Ministry of Magic? says that "whether Floo powder recognises Parseltongue is dubious/doubtful." If Parseltongue doesn't work, do other human languages, or are they out since it seems to be a Britain-only network?
But even if only English works, what about regional accents in English? As this video demonstrates, you might be speaking clearly in your accent and yet be totally unintelligible to someone with a different accent. Do you have to speak well-enunciated Queen's English, or is not-mumbling enough?