Aside from changing the Navigation bar, this is how we customized the ISBN message that is used in the 'Book Sources' special page e.g. [[Special:BookSources/1594200068]]. All these messages are in the '''MediaWiki''' namespace. So, if the message key is 'Requestaccount-acc-text', then you can change the content of the message by editing 'MediaWiki:Requestaccount-acc-text'.
Another option for finding the right message string, besides the 'uselang' hack, is to use the Translate Wiki (which is part of the system that provides translations for multiple software projects including MediaWiki). Visit https://translatewiki.net/wiki/Special:SearchTranslations and simply enter the "string you want to change". For example, if you're using the Special:UploadWizard and want to change the wording on the button that says "Select media files to share", you'd enter that phrase at Translate Wiki. Click 'search' and then in the results, click the link that says "Require all search words" (or add <tt>&match=all</tt> to the querystring). Presto, you've found that it's the <tt>MediaWiki:Mwe-upwiz-add-file-0-free</tt> string.
== How to make your own skin ==