Something like this,

var a = {en : 5,fr: 3,in: 9}
var b = {en: 8,fr: 21,br: 8}

c = {}
for(var k in a){
if(typeof b[k] != 'undefined'){
  c[k] = a[k] + b[k]
  
}
else{
 c[k] = a[k]
}
}