> If the entry is in English, and the style demands sentence case, convert to sentence-case and output

Nope: not possible to automatically determine which capitalised nouns are proper (and thus remain capitalised in sentence case) and which are common (and thus become uncapitalised).

This is in fact why it is better to store sentence case: it can be unambiguously converted to title case while the reverse is ambiguous. It’s not mere preference.