Including Persona 5 on this list might be controversial. However, there's no denying that good though the game undoubtedly is, it falls far short of Atlus's standards in localization and what fans have come to expect from the company. That departure from the high-quality norm is a bigger gaffe than any translation awkwardness in the game.
Fortunately, for the most part, the game's dialogue quirks don't come anywhere near Kitty Love and Hollow Fragment levels of bad. You do have to pause for a moment and consider what's being said from time to time, though.
Morgana is pretty quirky to begin with, so at first glance, this seems like just another manifestation of that personality. But the sentence doesn't technically make sense. "seriously trying to kill us" maybe or "serious about killing us," but serious to kill us is what you'd expect from an inexperienced translator or an early ESL student.
Conner Kramer put together a site listing some notably egregious errors (and getting some flak for it from the fan community as well), and he added some alternatives for a few of them. Here's an example:
His revision is a lot more like what fans got in Persona 4 and much more in keeping with the character doing the speaking as well. One would expect a high school principle to say something like "misdemeanor is not tolerated..." as opposed to "you will behave yourself," which is better suited to an elementary school setting.
There are other signs of carelessness too.
Image via j-entranslations
Persona games rely heavily on good dialogue to push the story forward and keep players interested. These issues are hardly game breaking, but they do break the immersion, which makes it difficult to remain invested.
What lies behind the issue is a mystery. It's possible some elements of localization were a rushed job, since the game was delayed to begin with. But it's equally possible it was simply oversight.
Yu Namba, senior project manager at Atlus and responsible for a good deal of Persona games' localization processes, once said he couldn't account for everything that happened, but tried to make sure the core narrative was coherent and clear. Other things could slip through the cracks, as they apparently did for P5.