Final Fantasy isn't a video game series, it's a religion. And like all great religions there are some rather large issues of translating from the original language to a new one. It used to be a lot worse, as if the people tasked converting the Japanese dialogue to English didn't really have a grasp on either of those two tongues.
Sometimes it's just a hilarious omission, such as a boy in X that meant to say, "I'm gonna be a blitzball player when I grow up," but instead says, "I'm gonna be a blitzBALL when I grow up." That one even has its own Facebook. However, in IV the bard really was spoony. That was just three Japanese translators being weird.
Most of the time the missteps in language don't really add up to much, but sometimes they can change the whole meaning of characters and stories. These are the five that had the most impact.