Function sync_existing_entries 

pub fn sync_existing_entries(
    source: &[Resource],
    target: &mut [Resource],
    options: &SyncOptions,
) -> Result<SyncReport, Error>

Updates entries in target using values from source.

Rules:

• Only existing entries in target are considered.
• Match by key first.
• Fallback match by translation in match_language.
• Never adds new keys to target.