JSFiddle - React, Tailwind, and code Playground

HTML

<form id="myId" name="myForm">
<input type="checkbox"  id="lang_1" value="de"/> DE
<input type="checkbox"  id="lang_2" value="us"/> US
<input type="checkbox"  id="lang_3" value="ru"/> RU
<input type="button" onclick="Select();"  value="Select all"/>
</form>

JavaScript

var inputs = document.querySelectorAll("input[type='checkbox']");
for(var i = 0; i < inputs.length; i++) {
    inputs[i].checked = true;   
}