I use the citation manager Zotero a lot. It's necessary to store all the titles of journal articles and books in sentence case (e.g. "Issues regarding the economy of France") because some publications require citations to use sentence style (remains unchanged) while others require title style ("Issues Regarding the Economy of France").

And obviously the solution cannot be naive, but is language-dependent, so that in English words like "the" or "in" don't get capitalized. And as the rules for titles are obviously language-dependent, so it goes without saying that the algorithm would have to be localized.

(Note that while it's relatively trivial to convert from sentence case to title case in English, it's impossible to automate in the opposite direction, because you never know if a capitalized term in the title is a proper noun or not.)