OOZ662 about 4 years ago It's a decently common way to see her name spelled, as it's closer to how the Japanese would pronounce "Flandre." That is, like "frahnd-lay." I seem to recall it might be from a different romanization method too, but I know very little about that.
NNescio about 4 years ago /flã.dʁ/Roughly, "flon-dr". (the a is combined with the n, resulting in a nasalized 'a' that sounds sort of like "o" to an English speaker.)