[FIX] website: use default website lang for configurator

The configurator introduced at [1] uses a frontend controller to
display its content. However this results in inconsistent language
translation as the python code uses the users's language but the
localization services uses the frontend's language when on a frontend
page.

Steps to reproduce:
- Create a database with the only language being french
- Install website
- In the configurator, all the pages will be in french except
  "Pages and Features"

With this commit, the language for the configurator will always be
the language of the website it's configuring. A test also make sure
that this is the current behavior.

[1]: https://github.com/odoo/odoo/commit/e8a5af2e28a4724f86783746706cb59175f086bf



task-2687416

closes odoo/odoo#80055

Signed-off-by: Romain Derie (rde) <rde@odoo.com>
Co-authored-by: JKE-be <jke@odoo.com>