"i18n-keyless does not support plurals or genders"

It does. Pass count and one source form ({count} items), and the model writes every CLDR plural branch each language needs — French 2, Russian 4, Arabic 6 — validated against Intl.PluralRules before they are saved. Ordinals (1st, 2e) and gender/select work the same way. Guide: Plurals and genders.

import { I18nKeylessText } from "i18n-keyless-react";

function CartLine({ count }: { count: number }) {
  return (
    <I18nKeylessText count={count}>
      {"{count} articles dans votre panier"}
    </I18nKeylessText>
  );
}

Zero-specific display text ("Your cart is empty" instead of "0 items") is a conditional in the component — that is display logic, not a plural rule.