1. Firstly, download XML file from here
2. Open strings.xml in your editor of choice (e.g. Notepad++)
3. Translate and keep in mind these important things.
Obey the XML format. So, <string name="do-not-change">ONLY TRANSLATE HERE</string>
Don't translate lines which contain translatable="false".
There are some characters which must be escaped in translations.
Don't add extra spaces or periods. Don't delete existent ones.
4. Once you finished translating, save file named like values-<language code>, were <language code> is 2 letter ISO code of the language. For example values-es for Spanish, values-fr for French.
5. Translated XML file send to firstname.lastname@example.org