localeCallback add access to request

New Feature / Enhancement Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.

Current Limitation

Currently choosing localization template requires writing user's locale to the user table and then querying it before sending the email. This is done in the localeCallback. However, this requires 1 read operation each time the email is dispatched. In addition, this doesn't allow for automatic locale detection. And if user changes browser language, they will still be getting their emails in their old language.

Feature / Enhancement Description

Would it be possible to provide access to request object inside the localeCallback? The request object of cloud functions usually contains the Allow-Language header, or cf-ipcountry header for the sites behind cloudflare.

These can be conveniently used for dynamic localization without any need to write or read anything from db.

Example Use Case

    localeCallback: async (req) => {
      const locale = req?.headers["accept-language"]?.split(",")?.[0]?.split("-")?.[0]; // en or tr or es
      return locale;
    },

Alternatives / Workarounds

3rd Party References

Start by locating the localeCallback entry point and tracing how the adapter invokes it during email dispatch. Confirm what request data is available there, then make the callback receive the request so the shown Accept-Language example can select a locale without a database read; verify existing localization behavior remains intact.