The persona prompt was built from the raw OUTPUT_LANGUAGE env while the
reply-language directive read the settings-web UI value (config JSON), so
changing the language in the settings page was honoured by the directive
but ignored by the persona, leaving them contradicting each other.
Add _resolve_output_language() as the single source of truth (config wins
over env) and feed the same resolved value to both build_system_prompt()
and reply_language_directive(). Update docs/llm_contexts.md to match.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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