I am trying to make a simple xml converter for TMX format using xml.etree. How do I create the language attribute that looks like this: `xml:lang`. 

What I tried:

```
    root = et.Element("tmx")
    body = et.SubElement(root, "body")
    tu = et.SubElement(body, "tu")
    tuv_en = et.SubElement(tu, "tuv", xml:lang="en")  # 'xml:lang' gets error
    tuv_zh = et.SubElement(tu, "tuv", xml:lang="zh")
    seg_en = et.SubElement(tuv_en, "segment").text="en text."
    seg_zh = et.SubElement(tuv_zh, "segment").text="zh text."
    tree = et.ElementTree(root)
    tree.write( file + ".xml")

```

What I want: 

```
<tmx>
  <body>
    <tu>
      <tuv xml:lang="en">
        <seg>en text.</seg>
      </tuv>
      <tuv xml:lang="zh">
        <seg>zh text.</seg>
      </tuv>
    </tu>
  </body>
</tmx>

```