JSFiddle - React, Tailwind, and code Playground

HTML

<img id="myFlag" src="flag_default.png"/>

JavaScript

var supportedLangs = ['de', 'en', 'cn'];
var userLang = navigator.language;
alert(userLang);

if($.inArray(userLang, supportedLangs) >= 0){
    $('myFlag').attr('src', 'flag_' + userLang + '.png');
}