Answer
Use Translation Manager, not the source Field Description itself: go to Field Description Translations, search using the Field Description RTF (the English help text), the Translation Prog Text (the English field label), and the target Lang code, then select the broken translation record and choose Reject Translation from the right-click menu. From there you can correct the translated text and save it. After fixing it, refresh the language cache so the corrected translation actually takes effect in the client instead of the stale/broken cached version.