Note: This feature is only available in Web Workers.
The WorkerNavigator.language read-only property returns a string representing the preferred language of the user, usually the language of the browser UI.
A string representing the language version as a BCP 47 language tag, for example en, en-US, fr, fr-FR, es-ES, etc.
Note that in Safari on iOS prior to 10.2, the country code returned is lowercase: "en-us", "fr-fr" etc.
You can run this insert a web worker:
if (/^en\b/.test(navigator.language)) {
doLangSelect(window.navigator.language);
}