You should always store your titles in sentence case in Zotero, then put any proper nouns in uppercase. If you need a Word to always be lowercase, wrap it in <span class="nocase"> </span> Tags

See https://www.zotero.org/support/kb/sentence_casing — there's simply no way to reliably automate a conversion to sentence case.

We could obviously provide a way to batch-convert titles to sentence case, but I'm not sure it makes sense, given that you need to correct any proper nouns, and there's a good chance some/many of your titles are truncated in the middle pane and wouldn't be fully visible.

(For what it's worth, I've wondered before whether it would make sense to bring an auto-conversion feature like we have in ZoteroBib to Zotero, but it'd be a bit more dangerous without the kind of explicit review step we have in ZoteroBib. Data can also get into a Zotero library in so many ways that it might get confusing. For example, we'd want it for saving from the web or Add Item by Identifier, but I'm not sure we'd want it to apply to imports from a file, but if you click a RIS file in the browser and the connector intercepts it, does that count as saving from the browser or as an import? Joining a group library obviously doesn't convert the items in that library, but if you drag an item from a group library to your own library, does it get converted? Also worth noting that in ZoteroBib it's only a soft conversion — if you change back to another style, it goes back to its original form, unless you made a manual edit in the meantime.)

At your own risk:

1. Go into Tools -> Developer -> Run Javascript
   
2. Check "Run as async"
   
3. Paste in the script below, verbatim
   
4. select some items in the main zotero pane
   
5. click "Run"
   

const sentenceCase = Zotero.Utilities.sentenceCase || function (val) {
newVal = val.toLowerCase().replace(/\s*:/, ":");
newVal = newVal.replace(/(([\?!]\s*|^)([\'\"¡¿“‘„«\s]+)?[^\s])/g, function (x) {
return x.replace(/\s+/m, " ").toUpperCase();});
return newVal
}
let title
for (const item of Zotero.getActiveZoteroPane().getSelectedItems()) {
if (item.isFeedItem || !item.isRegularItem() || !(title = item.getField('title'))) continue
item.setField('title', sentenceCase(title))
await item.saveTx()
}