Detect the preferred language of the current user

Use Navigator.language or the first value of Navigator.languages if available, otherwise return defaultLang. Omit the second argument, defaultLang, to use 'en-US' as the default language code.

const detectLanguage = (defaultLang = 'en-US') =>
  navigator.language ||
  (Array.isArray(navigator.languages) && navigator.languages[0]) ||
  defaultLang;

// EXAMPLE
detectLanguage(); // 'nl-NL'

Source

https://www.30secondsofcode.org/js/s/detect-language