The inherent conflict between strict lexicographic sort keys and analogical or bibliotechnical sort keys leads to recurrent debates on the wiktionary projects (and possibly others). The ability to specify a different sort key for each category helps, but there are remaining instances where the most helpful solution would be for the MediaWiki software to offer some means of inserting any given page in two (or more) places in a category. Maybe through a magic word such as DEFAULTSORT2. This could be extended to all categories by allowing each category to accept multiple piped parameters (e.g. [[Category:Some category name|first sort key|second sort key]]).
To give just one specific example out of many, on the French Wiktionary a page like ''Le Caire'' (i.e. Cairo) would benefit from appearing in the category ''Noms propres en français'' (translation: Proper nouns in French) twice, once under C and once under L.