Heh, a very curious question.
It's a huge topic, but very briefly:
Andre, while it's not exactly a language issue, in practice, the implementation of the sidebar does involve system messages, so it is a good idea to involve the Language team in finding a good way to make them more robust and portable.
A quick and simple solution is probably something like this:
- Discuss with people from other Wikisource editions what do they want in the sidebar.
- Once you arrive at an agreement for at least a few of them, write the common sidebar code. By "code" I mean something like s:en:MediaWiki:Sidebar, so it's something with which you are already familiar.
- Once it's done, it can probably be added as a message in Extension:WikimediaMessages. If you don't want to go deeply into Gerrit, I can do it for you.
- Once it's merged, it will be possible to translate it on translatewiki.net, and after it's translated, it will be auto-deployed to all the languages. Doing this is more robust than managing a local MediaWiki:Sidebar in each wiki site.
- So far, so good, but I'm less sure about the next part: Each language will have to get rid of its local MediaWiki:Sidebar (this is easy—just delete the page) and to enable the sidebar code from WikimediaMessages. I'm not quite sure how to do this. Maybe User:Nikerabbit will have an idea. And maybe he'll suggest scrapping this whole plan, and doing something else ;)
In the longer term, I envision proper product management work around the sidebar: Analyzing its usability, design, clicks frequency, customization options, and so on. But the naïve little plan above can be a first step.