Translating Semantic UI Docs
You can sign up for our project translation at the project translation hub on Transifex
We use Transifex to coordinate between multiple developers who've decided to contribute time to help with Semantic translations. Its really helpful and completely free for open source
After registering, you should be able to start translating using the "Live" button shown here
Transifex Live is an editor that lets you translate strings while browsing through the contents of the site, so you can understand the context of each text string. 
In addition in this menu you can leave comments for other translators.
If you are unsure of a translation, save it but don't mark it as "reviewed".
I advise to try translating the most important strings first like the homepage headings, and the UI definition names, before going too far into other page text.
One other note. There may be strings in the translation dictionary which are out of date, each new piece of text content must be individually accepted or ignored, so I recommend only translating strings visible in the site, or listed in the "content" of the page. If you see something important that you can't translate, feel free to shoot me an e-mail so I can fix it.
The goal is to try to distribute the translations between as many people as possible, to lessen the burden on any individual, so if you don't have much time available that's ok! Every little bit counts!
If the language you would like to translate to does not exist yet, you can request it by clicking the Request language button.
If you need help using Transifex, you can go to their documentation page