[Feature Request] Add locale setting for system-generated content language

Bug Description
Title: Feature Request: Add language/locale setting for Ctrl+E permission explanation

Body:
Ctrl+E permission explanation always generates in English.
SessionStart hooks can set conversation language, but Ctrl+E explanations don't inherit it.

Request: Add a locale setting in settings.json to control the language of
system-generated content like Ctrl+E explanations.

Example:
{ "locale": "ja" }

Environment Info

• Platform: darwin
• Terminal: Apple_Terminal
• Version: 2.1.59
• Feedback ID: 66e0f444-32d4-4ca6-84b6-34b704aaeda2

Errors

[{"error":"Error: Plugin vercel has an invalid manifest file at /Users/iwasakihayato/.claude/plugins/cache/claude-plugins-official/vercel/1.0.0/.claude-plugin/plugin.json.\n\nValidation errors: : Unrecognized keys: \"category\", \"source\"\n\nPlease fix the manifest or remove it. The plugin cannot load with an invalid manifest.\n    at M01 (file:///opt/homebrew/lib/node_modules/@anthropic-ai/claude-code/cli.js:2006:11445)\n    at async bO7 (file:///opt/homebrew/lib/node_modules/@anthropic-ai/claude-code/cli.js:2014:356)\n    at async e4Y (file:///opt/homebrew/lib/node_modules/@anthropic-ai/claude…

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