Heroist
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Well, Heroes player, you think it's unacceptable to have informal expressions and slang in the characters' speech in games. But the point is that few real people speak exclusively in formal language, so it's strange to demand this from computer game characters. After all, you want immersion, not for them to sound like some kind of robots, right? And it is precisely a more "human" style of speech that gives characters individuality (although for some other character, the same individuality would be given by an official style of speech). Often, this is precisely why a unique slang is introduced in the universe of a game, which many NPCs use in their speech.
Again, if we're talking about dialogues, it's normal and acceptable.
if it's done specifically to show the character's personality
Whatever