Following on from the fix of [T202424]:
Some system messages in the MediaWiki: namespace are wikitext, and accept wikitext features like links and images. Their content model is Wikitext. That is correct.
Other system messages in the MediaWiki: namespace are plain text messages. If you add wikitext to them, it just shows up as code. Their content models are inaccurate; they should be plain text, rather than wikitext.
A good example is MediaWiki:Titlewhitelist.
Some of the others are very short system messages that just say "submit" or "edit" or something. Most of the longer messages tend to be Wikitext, but it's all a bit inconsistent due to the way that the software has been written.
Two steps here:
- Identify non-wikitext system messages
- These should be a plain text content model by default on new installations
- Existing wikis may need something to convert them to the correct format.
I appreciate that this may not be a priority. Perhaps the obvious ones should be done first, and this left as an ongoing open task.