Update: I built a small free beta so the idea can be tested as a working tool rather than discussed in the abstract:
https://hickery68.itch.io/localization-change-review
It is a single-file offline browser tool for wide CSV files with stable keys. It flags source-changed / translation-unchanged rows for human review, plus added and removed keys. It does not translate, modify files, or replace gettext/PO workflows that already provide reliable fuzzy flags.
If anyone tries it on a real CSV workflow, I’d especially value feedback on whether the report catches something your current process misses.