Oh never mind! the 'available translations' bar DOES show up when I switch to English. So the only question that remains is, why doesn't the mediawiki switch to the available English translated version by default?

Hmm, the translations ARE done correctly. However, the language chooser doesn't change the current page to the language that is selected. I added
<languages /> to the top of the dutch original page. Now I see that I can select English at the top of the page. When I click that, the page is translated to English using my previously given translations. However, switching to the English language does not automatically 'switch' to the english version of the page. Is this intended behaviour? because, that seems like a bad default (as opposed to the mantra of 'good defaults'). Ah well, this is already a lot better then it used to be..

Yeah that would be great!
The Translate extension from the MLEB extension bundle is working, I can translate pages, however when I switch from Dutch to English for example, the translations do not show up. I'm not getting errors either... weird.

Both rebuilding the cache and excluding only the require_once line for Babel did not work...

After applying the 4 changed lines manually, still the same error:

After spending more then half the day updating mediawiki (!) I'm getting exactly the same error:

Thanks. It did not work unfortunately, our mediawiki was not up to date so the line numbers differed. However I was able to find the same piece of code and I added the green lines (those that were added in the patch). I restarted the mediawiki web installer but I got the same error.

Okay thanks, will try to do that. But how can I download the patch as a set of files? Unfortunately, our wikimedia is not a git clone from the main wikimedia repository so I cannot git pull in the patch?

Can someone explain in user talk how to solve this? I'm trying to install MLEB using this
https://www.mediawiki.org/wiki/MediaWiki_Language_Extension_Bundle#Installation
Guide and getting the same error.
We are using mediawiki as an information system but I'm not really familiar with its intrinsics nor do I want to be :)
I've tried running some scripts, namely

maintenance/update.php
maintenance/rebuildLocalisationCache.php --force