How can I determine which language is being updated when handling the CatalogEventBroadcaster.LocalEntryUpdated?
LocalEntryUpdated is raised without language-specific. Whenever you save a CatalogEntryDto, it'll be raised, and CatalogEntryDto is language-agnostic.
So what's the problem you're trying to solve?
When an catalog entry is updated (saved / updated) I need to re-index it in the appropriate language index, so I need to know the language of the update / insert.
I don't think you can index (the search index) by language, the language is per-entry, not per-language.
It's a custom index