Register Is Harder Than Vocabulary
A reader flagged one line in my second novel as the moment she stopped believing the police officer, and the line had no wrong words in it, only the wrong address form.
The officer was questioning a nineteen year old suspected of nothing worse than being present, and I had written him using "vous," the formal address, throughout. My reader, who had grown up around actual police stations in Marseille rather than around my idea of them, told me no officer talks to a nineteen year old that way unless the boy's father is a magistrate. The formal address signalled respect the scene did not intend and the character would never have offered. I had translated the scene correctly, word for word, and gotten the officer's whole social position wrong.
The register ladder
Vocabulary mistakes get caught. A dictionary catches them, an editor catches them, sometimes a spellchecker catches them. Register mistakes do not announce themselves, because every word in the sentence is correct. What is wrong is the rung of formality the sentence is standing on relative to the two people in the scene, and nothing on the page tells you that rung is wrong except a reader's discomfort they usually cannot name.
I now think of register as a ladder with about five rungs a working sentence can sit on: intimate, familiar, neutral, formal, and official. The mistake is not picking the wrong rung once. The mistake is not tracking that every relationship in a scene has its own ladder, and that the same character can be three rungs apart with two different people in the same paragraph. My officer needed to be neutral or familiar with the boy and formal with the boy's lawyer thirty seconds later, and I had flattened both relationships onto the same rung because I was translating meaning, not translating the ladder each character was standing on.
Why vocabulary is the easy part
You can look up a word. You cannot look up whether a Marseille police officer in 2004 uses "tu" with a suspect he considers a child and "vous" with the suspect's mother in the same room, because that is not a fact stored in a dictionary. It is a fact stored in the specific social world your scene is set in, and if you did not grow up inside that world, or interview someone who did, the register choice will read as invented no matter how correct every individual word is. My fix for the officer scene took an afternoon of asking someone who had actually been arrested at nineteen what the room sounded like, not what was said, but who was allowed to speak informally to whom.
The counter you hear from editors
The counter I hear most from editors trained on English manuscripts is that this is a French problem, that English does not have a formal and informal address form built into its grammar the way French does with "tu" and "vous," so the whole issue evaporates once you write in English. That is true at the level of grammar and false at the level of the sentence. English marks the same ladder through different means: first names against titles, contractions against full forms, "yeah" against "yes," a dropped subject against a complete one. An English scene where a boss says "Could you possibly get me those figures" and one where a boss says "Get me the figures" are standing on different rungs of the exact same ladder, no formal pronoun required. English hides the rungs more carefully than French does. It does not remove them.
What tracking this actually buys you
Once you treat register as its own variable, separate from vocabulary, you start catching a category of error that used to hide inside otherwise clean prose. You read a scene and ask, for every pair of characters in it, which rung each one is speaking from, and whether either of them shifts rungs mid conversation on purpose or by accident. A shift on purpose is one of the sharpest tools a scene has, a character suddenly speaking formally to someone they usually address as family is a small earthquake a reader feels without being told why. A shift by accident is just a mistake wearing the costume of correct grammar, and it will cost you exactly one reader's trust, quietly, the way it cost me mine.