Translation Editor
What it is
Every piece of text the CMS itself generates — buttons, labels, messages, dates — is translatable, and you can even set your admin panel to one language while your public site runs in another. Useful if, say, you write and manage your site in your own language day-to-day, but your site's actual visitors read a different one.
Set both from Settings → General. If you don't set an admin language, it simply follows whatever your public site's language is set to, so there's nothing extra to configure if you don't need the two to differ.
Editing translations yourself
Open Tools → Translations to see every piece of text next to its current translation, with anything missing or incomplete highlighted so you know exactly what still needs attention. Edit as many as you like, then save them all at once.
Adding a language that isn't there yet
If your language isn't available yet, New Locale creates it for you — starting from the full English reference so nothing gets missed, ready for you to translate at your own pace rather than needing everything done before you can start.
This only covers text the CMS itself generates. Your actual articles, pages, and projects are written in whatever language you type them in — translating your own content into another language means writing a separate version of it yourself, the same as you would on any site.