Thanks to T179402, it is now possible to know more complete and useful information about cases when people didn't find the language that they searched for in compact language links and elsewhere.

Now that the information is available, it must be presented conveniently so that the editors community will be able to learn useful things from it. The first step is https://gerrit.wikimedia.org/r/#/c/413270/ , but it needs improvement.

The questions that the report must answer:

• Which articles do people fail to find most often? The query will be: which articles did the largest number of unique users searched for and failed to find during a given period (probably a week)? It is easy to automate the publishing of the info, but community members will have to analyze the results and understand why couldn't the article be found. The reasons that people fail to find an article include, but are not limited to the following:
  • Volunteer editors never wrote the article. Solution: write the article :) Seeing that an article is actually in demand should be a good indicator for people who decide which articles they want to create.
  • The article had existed, but was deleted. Solution: restore the article, or leave it deleted (deletion is not necessarily bad).
  • The article exists, but was not linked correctly using Wikidata. Solution: add the correct link.
  • A relevant article exists, but couldn't be linked, because wikis in different languages have different ontologies and Wikidata allows only one link. For example, English Wikipedia's Walnut is only about the nut, and there's another article about the tree, but in Wikipedia in many languages it's the same article. Solution: In some case, reconsider the ontology. But perhaps a smarter design and engineering solution can give a more comprehensive result. For example, allowing something like "see also" interlanguage links. We can start to think about such a comprehensive solution after this task is resolved and data for at least a few weeks is analyzed qualitatively.
• In which contexts (not necessary compact language links) do people search for languages and fail to find them? Why? Possible reasons can be:
  • Problematic design of the ULS panel itself, so it doesn't clarify that searching for this language is irrelevant in this context.
  • Incorrect initialization of ULS in that context. (For example, incorrect languages option in $.uls().)
  • A language name that is not included in the API. See T186781 and T178996 for examples.
  • Problematic design of the wiki page on which it is used. (List of Wikipedias comes to mind.)