[FIX] web: user lang has the priority on partner lang

Potentialy, the timezone too.

On item action click (such as menus in More.. and Print..), the data in form view had the priority on user context (through the sidebar_eval_context)

Therefore, if a field "lang" was present in the form view (like in partner form), the web/action/load xmlrpc call was using the partner language instead of the user language.

Example of wrong use case before the fix:
 - Set the user language in French, then go to a partner form of a partner with English set as language
  - Click on any button of the partner form, such as the "Invoices" button, notice that the last item of the breadcrumb is in English, instead of Frenh (the user language)
  - Click on any menu opening a wizard in the More.. dropdown menu, notice that the wizard title is in English instead of French
  - Print any report from the Print dropdown menu, notice that the report file name is in English. If you print the same report for the same partner but from the list view, the report file name is in French.