The redirects table in Module:Language/data connects Wikipedia language codes to the corresponding code used on the English Wiktionary. wikt:Wiktionary:Language treatment records this relationship for ISO codes. For instance, all of bs (Bosnian), hr (Croatian), sr (Serbian), cnr (Montenegrin), kjv (Kajkavian) are placed under the header for sh (Serbo-Croatian) in Wiktionary entries. See for instance wikt:kaj#Serbo-Croatian, the word that Kajkavian is named after. The subsumed codes should still be used in language-tagging on Wikipedia.