First of all, let me tell you how nice it is to hear that more people are willing to contribute to Whonix by translating, that's great.
Now, regarding your questions:
For a guide on how to make a page "translatable", please regard this: https://forums.whonix.org/t/howto-preparing-the-wiki-for-proper-translation-and-translating-homogeneously/1450 Furthermore, I'd need your username on the wiki, so I may give you the necessary rights to do it.
Just looked at the site and was baffled to say the least. As you've mentioned, the translation shows a completley different original to what is actually to be found on the download page. This seems to be a bug caused by Mediawiki (the software our site currently is based on) which makes me strong in my believe that we should ditch it as fast as possible as working with it is unreasonably complicated.
Here's what, at least from my judgement, has happened:
The download page received a major facelift not to long ago making it more simple and user friendly. Since it however had been translated in the past it just kept the old template instead of using the new one, for some unexplainable reason.
After applying the necessary translation tags again, everything seems to work, although old versions of the translation now suffer under the "old look".
Regarding the download page though, there are some additional things I need to mention:
The box you see actually isn't a part of that page but rather an element loaded from another page, namely this one: https://www.whonix.org/wiki/Template:Supported_Platforms_Table
So, to translate it, you first have to translate this site, than create a language of your choosing on the download page, and link to that newly created table in your translation.
This, as you can see, is quite a lot of work for what should amount to a quick rewrite, which is why I'm currently working on creating a new webpage for Whonix, which aims to not only look better, but also be more accessible for translation.
That's the reason why, while I'm very thankful for you wanting to help, I'd like to propose maybe waiting a few weeks because by than, Mediawiki should be history, at least if everything goes by plan.
Have a nice day and thank you for your willingness to help,