feat: add i18n support with react-intl and initial translations

[ckemmler]

This PR adds internationalization (i18n) support to the Agent Chat UI project, making it easy to localize the application into multiple languages.

Changes Included:
• Integrated react-intl for internationalization.
• Created a global I18nProvider to manage locale switching dynamically.
• Added a basic LanguageSelector component to allow users to switch languages client-side without page reloads.

Created initial translations for the following languages:
• English (en)
• French (fr)
• Dutch (nl)
• Portuguese (pt)
• Spanish (es)
• Italian (it)
• German (de)
• Russian (ru)

Translated the following UI strings:
• “Send”
• “Hide tool calls”
• “Thread history”
• “Type your message”
• Default locale is set to English (en).

Why?
• Improves accessibility for non-English speakers.
• Prepares the application for wider public and international deployments.
• Provides a scalable structure for adding more translations later.

[vercel]

@ckemmler is attempting to deploy a commit to the LangChain Team on Vercel.

A member of the Team first needs to authorize it.