Let's workout a plan to let the community know our rollout plan for BM25.
Email sent to wikitech-l, wikitech ambassadors, and discovery email lists:
Latest search updates
After extensive testing over the last several months using a new search query scoring method called BM25 (Best Matching) , we recently completed a limited production release to the following top languages: English, German, Spanish, Russian, Portuguese, French, Italian, Polish, Dutch and Arabic. This new release is replacing the older search method called tf-idf (term frequency-inverse document frequency) .
We have additional testing to do [3,4] to figure out if BM25 will work in languages that don’t use spaces in-between their words, i.e.: Japanese, Chinese, etc.
The Discovery team announces much of our completed work in weekly status updates [5, 6], but some of the work isn’t actually obvious to anyone who uses our search engine - that is because it isn’t actually ‘live’ until a complete re-index of the servers occur. We’ve created a recurring ticket in Phabricator  to keep track of the work that goes live in production after a re-index, such as the one we’ve also just completed. A few highlights of the recent re-index are implementing ascii-folding for the French language and fixing several bugs for French ÿ, and Russian ’Е’ and 'Ё' when those characters are entered in a search query.
Cheers from the Discovery Search Team!