When searching on commons we take account of the interface language to improve search results, but for non-logged-in casual searchers it's not obvious that they can switch languages
This could be improved with a CTA on the search results page (or something similar) - perhaps a little box that says "You're searching in English. To search in a different language choose from this list"
I work and train people that are frequently working in multiple languages, and I get the impression that folks who do not have English are unlikely to discover the language change if it stays behind the registration wall. Moreover, multilingual users have very few hints that different language interfaces for the search, change the outcome of the search (i.e. using a spanish language term in the English search doesn't leverage structured data to also include other language terms, whereas using the English language term would).