Detect language changes when switching to app. I personally recommend setting current language only on app launch. Reacting to changes later benefits only a small set of users, focus on the 99%!

language-detect.js

// Example code, use with caution!
// Events and .hidden property requires no prefix since 18

// Set current language on launch
document.documentElement.lang = navigator.language;

function visibilityChange() {
	// user went off page, ignore
	if (document.hidden) return;

	// get current language and compare it to old one
	var language = navigator.language;
	var element = document.documentElement;

	if (element.lang == language) return; // no change

	element.lang = language;

	// propagate change with an imaginary event (implement your own)
	var evt = document.createEvent('Event');
	evt.initEvent('languageChange', true, false);
	evt.language = language;
	document.dispatchEvent(evt);
}

// Add event listener.
document.addEventListener('visibilitychange', visibilityChange, false);

This code should probably not restrict the language to a substr, but the rest sounds like a feasible approach.