If you want to translate a page you can click on the language you want to translate to at the top of the article, that will take you to a version of the wiki in that language. If the translation for that language doesn't exist yet you can click on the pen icon floating on the right side of the page to create it. That will take you to the article editor. After you are done translating you need to complete the captcha (click the white box next to “I'm not a robot” and wait until it turns into a green checkmark). Then enter a summary of the changes you have made to the page and click save. You can also use the preview button to see what the article looks like on the website before publishing it. You don't need to complete the captcha for that.
Only administrators of the wiki can currently add new supported languages to the wiki. If you want to translate a page to a language that isn't supported yet you can send an e-mail to email@example.com and I will add it as soon as I read the e-mail.