Agent reasoning should respect user's language

[AmirTlinov]

What feature would you like to see?

Problem

Agent reasoning in TUI is always displayed in English, regardless of the user's input language. This breaks the narrative flow for non-English users.

Impact

• Cognitive load: Users must mentally switch between languages while reading reasoning + writing prompts
• Lost context: The reasoning is the primary way users understand agent decisions — if it's in a foreign language, this value is diminished
• Inconsistent UX: User writes in their language → agent reasons in English → creates jarring experience

Proposed solution

Add language detection for user input and instruct the model to output reasoning in the matched language.

Alternatively, expose a reasoning_language config option in config.toml:

[model]
reasoning_language = "auto"  # or "en", "ru", "zh", etc.

Prior art

Claude Code handles this gracefully — reasoning appears in the language the user communicates in.

Additional information

Tested on macOS with Codex CLI. User input in Russian, all reasoning output appeared in English.