TLDR: Can’t we please use stemmers matched to site languages?
According to this post from Nick Craver and this post from Haney, our searches are using the ElasticSearch stemmers. The problem is that stemmers have to be tuned for each particular language being used, or they generate too many false positives and false negatives. We should please use these, because they appear to be available.
This problem is especially notable in the various non-English incarnations of Stack Overflow:
In Spanish, the plural of error is not errors as in English, but errores. However, on the Spanish-language encarnación of Stack Overflow, searching for the plural errores produces results very different from searching for the singular error. That is incorrect because these are the same lemma.
In Portuguese, the word for a “function” is função in the singular, with the perfectly regular plural inflection funções. However, on the Portuguese-language encarnação of Stack Overflow, searching for the singular função produces results crazily different from those obtained when searching for the plural funções there does. This is again wrong because those should resolve to the same lemma, as before.
In Portuguese, the word for “variable” is variável in the singular, with the perfectly regular plural inflection variáveis. However, searching for the singular variável produces somewhat different results than searching for the plural variáveis produces.
This is much worse when you have verbs. English gets to piggyback some of its singular–plural verb inflections atop noun inflections (think drive, drives), but more strongly inflected languages cannot do that. It’s not reasonable for people to have to think about all the tenses, aspects, persons, and moods just to find hits for the verb “return” in its many possible forms.
This same problem also surfaces in creating new tags, because the duplicate-of-singular/plural checker completely misses tags whose shift in grammatical number triggers “more things in other tongues than are dreamt of English philology”.
Because the Spanish, Portuguese, French, German, Italian et cetera Stack Exchange language sites use tags in those languages, they’re always getting problems with tags that would be easily fixed by switching the stemmer.