next-i18next/dist/es/utils/lng-from-req.js

The easiest way to translate your NextJs apps.

export const lngFromReq = req => {
  if (!req.i18n) {
    return null;
  }

  const {
    allLanguages,
    defaultLanguage,
    fallbackLng
  } = req.i18n.options;
  const fallback = fallbackLng || defaultLanguage;

  if (!req.i18n.languages) {
    return typeof fallback === 'string' ? fallback : null;
  }

  const language = req.i18n.languages.find(l => allLanguages.includes(l)) || fallback;

  if (typeof language === 'string') {
    return language;
  }

  return null;
};