Feature: /insights report should respect user's language preference

Description

The /insights command always generates its report in English, regardless of the user's language configuration. Even when:

• CLAUDE.md explicitly states "Always respond in Russian" (or any other language)
• Language rules are defined in .claude/rules/
• The entire conversation history is in a non-English language
• Claude Code correctly uses the configured language for all other interactions

...the /insights report is still generated entirely in English.

Expected Behavior

The /insights report should be generated in the language that the user has configured via their CLAUDE.md or rules files, consistent with how all other Claude Code responses respect language preferences.

Current Behavior

The report (tips, metrics, session summary) is always in English. This creates an inconsistent experience for non-English users who have explicitly configured their language preference.

Suggested Solution

Apply the same language preference detection that works for regular responses to the /insights report generation. This could be done by:

1. Including the user's language instructions in the context when generating the insights report
2. Or detecting the primary language from the conversation history
3. Or adding a locale/language setting specifically for reports

Environment

• Claude Code CLI (latest version)
• Language configured via CLAUDE.md: Russian
• All rules and instructions in .claude/rules/ reference Russian language

Additional Context

This is a localization/i18n improvement that would benefit all non-English speaking users of Claude Code.