JSFiddle - React, Tailwind, and code Playground

JavaScript

let sample={
  "yuiwedw":{
  "id":"yuiwedw",
  "loc": "ar",
  "body":{
    "data":"we got this",
    "loc":"ar",
    "system":{
      "fname":"admin",
      "lname":"admin",
      "details":[
       {
        "id":"Admin_01",
        "loc":"ar"
       },
       {
        "id":"Admin_02",
        "loc":"ar"
       }
       ]
      }
    }
  },
    "hbjds324":{
  "id":"hbjds324",
  "loc": "ar",
  "body":{
    "data":"testing",
    "loc":"ar",
    "system":{
      "fname":"public",
      "lname":"servent",
      "details":[
       {
        "id":"public_01",
        "loc":"ar"
       }
       ],
       "Available":true,
       "loc":"ar"
      }
    }
  }
}


function replaceLoc(obj){
 	for(const key of Object.keys(obj)){
  	 if (typeof obj[key] === 'object'){
     		replaceLoc(obj[key]);
     }
     if (key === 'loc'){
     		obj[key] = 'en'
     }
  }
}

replaceLoc(sample);

alert(JSON.stringify(sample, null, 2));