The message searchmenu-new-nocreate is empty and ignored in core, to let other sites customize it.
On wikidata.org, it's customized in a way that shows specific strings depending on the namespace in which it's viewed, and transcludes translations made using locally translatable pages (using the Translate extension). It's a relatively good solution, because it allows all volunteer translators to contribute a complete localization. However, it has a couple of problems:
- Other sites that use Wikibase and WikibaseLexeme don't enjoy this. They either have to customize something of their own or copy the wikidata.org solution.
- These are essentially a part of the site's user interface, which are usually translated on translatewiki. Having to translate in several places is not so convenient, especially when there's no good reason to make the separation.
It would be much better if the overriding messages for searchmenu-new-nocreate were in the Wikibase and WikibaseLexeme extensions, so that they could be translated in one place and all Wikibase users could enjoy them.