I guess the question is self explanatory. In case no readily solutions are available, which would be the easiest way to implement the message (except from editing MediaWiki's skins and adding the code, which would possibly require redoing the job after upgrades)?
1 Answers
There's an extension, CookieWarning, which does exactly that. It shows a message to a new user, that the site uses cookies, which can be acknowledged by clicking "OK". You can also optionally configure a link ("More information"), which can lead to a full Cookie Policy document on your site (it can be a link somewhere in the Wiki or an external page as well).
You can, if you want, change the text that is showed to users by editing the interface message cookiewarning-info (https://yourwiki.com/wiki/MediaWiki:cookiewarning-info).
Disclaimer: My answer might be opinionated, as I'm one of the authors of this extension.
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