_translate_one translated [title, description] in ONE Interpreter call and keyed
the whole-post passthrough on the aggregate detected_lang (the FIRST item). So an
English title + non-English description detected "en" and marked the post handled,
leaving the description untranslated. Now each field is translated independently
(its own detected_lang / passthrough) and the non-target field is stored on its
own; translated_source_lang reflects the translated field's language.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>