sampleOfXmlConvertCategory.groovy

class XmlConvertCategory {
    static String toXml(Map input) {
        def sw = new StringWriter()
        def builder = new groovy.xml.MarkupBuilder(sw)
        builder.doubleQuotes = true
        builder.metaClass.recursive = { Map map ->
            map.each { key, value ->
                "${key}"(value in Map ? { recursive(value) } : value)
            }
        }
        builder.langs(type: 'current') {
            recursive input
        }
        sw.toString()
    }
}

def input = [
    key1: "value1",
    key2: "value2",
    key3: [
        "key3-1": "value3-1",
        "key3-2": "value3-2",
    ]
]

def expected = '''\
<langs type="current">
  <key1>value1</key1>
  <key2>value2</key2>
  <key3>
    <key3-1>value3-1</key3-1>
    <key3-2>value3-2</key3-2>
  </key3>
</langs>'''

use(XmlConvertCategory) {
    def result = input.toXml()
    assert result == expected
}