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JavaScript

var xml_string = '<?xml version="1.0" encoding="UTF-8"?> <Langs><dirs>  <string>uk-fr</string>  <string>uk-it</string>  <string>uk-pl</string>  <string>uk-tr</string></dirs><langs>  <Item key="af" value="Afrikaans" />  <Item key="ar" value="Arabic" />  <Item key="az" value="Azerbaijani" />  <Item key="ba" value="Bashkir" />  <Item key="be" value="Belarusian" /> </langs></Langs>';

var output = $.parseXML(xml_string); // <-------- using jQuery.parseXML

var array = []; // <----------------------------- list of key

$('langs>Item', output).each(function(i, e) {
  array.push($(e).attr('key'));
});

console.log(array);