To translate you just need to create a page with the same English original name adding / and two-letter code corresponding to your language as a suffix. For example, Russian translation for Doc:Getting_Started page is Doc:Getting_Started/ru and Spanish translation is Doc:Getting_Started/es. (for how to add pages have a look at the first page of the writer doc.
When you translating text in page body please don't change links - just left them point to English pages. Link automatically point to pages with proper language if those translations are exist and they point to English ones if there's no translation yet. You might want to use the alternative text if the english version just links to the page without alternative text!
Please don't forget to update your translations as the original English page changes. Don't add your own stuff to translated page - add it to English first.
Formal Or Informal?
If the language you are translating to allows for formal and informal ways of talkingt to the reader, we prefer informal over formal.