[BACKPORT] website: translation if website default lang is not en_US

Before this commit:
When website default lang was not en_US, the translation of fields* in the
web_editor would not be saved correctly.
Let's illustrate with fr_BE set as main lang:

  The web_editor's RPC calls does not send language in the context when editing
  a page (6ebcc06), but it does in translation mode.
  Note that editing is always performed in the website main lang, fr_BE in this
  case.
  Therefore:
  1. When EDITING:
    The backend suppose the language is en_US as there is no lang in the
    context (hardcoded everywhere).
    It then write in the record itself instead of the ir.translation as it
    should since fr_BE is a translation. (In Odoo, every lang is a translation,
    en_US is  always the main lang)
  2. When TRANSLATING (in en_US):
    It would also write on model as the web_editor send the language to the
    backend when in translate mode. But since en_US is recognized as the main
    lang (which is correct), it also writes in the model, overriding the
    incorrect fr_BE writing which was also wrote on the model.

This has been fixed in 10.0 with these 2 commits:
036ccbe
2955bf2

With these fixes, the lang is correctly added (by the backend) if the website
module is installed.
By ensuring the lang is correctly set, the ORM _write() function will process
the edition and translation datas as it should:

  - If it receive en_US, it will write on the model
  - If it receive another lang, it will write on ir.translation

This closes #10883, closes #22403

*Not the HTML/XML fields which are handled differently by the ORM/translation

addons/web_editor/models/ir_ui_view.py

@@ -35,12 +35,18 @@ class view(osv.osv):
     def extract_embedded_fields(self, cr, uid, arch, context=None):
         return arch.xpath('//*[@data-oe-model != "ir.ui.view"]')
 
+
+    def get_default_lang_code(self, cr, uid, context=None):
+        return False
+
     def save_embedded_field(self, cr, uid, el, context=None):
         Model = self.pool[el.get('data-oe-model')]
         field = el.get('data-oe-field')
 
         converter = self.pool['ir.qweb'].get_converter_for(el.get('data-oe-type'))
         value = converter.from_html(cr, uid, Model, Model._fields[field], el)
+        if context and not context.get('lang'):
+            context['lang'] = self.get_default_lang_code(cr, uid, context=context)
 
         if value is not None:
             # TODO: batch writes?

addons/website/models/ir_ui_view.py

@@ -228,3 +228,10 @@ class view(osv.osv):
                     'active': v.active,
                 })
         return result
+
+    def get_default_lang_code(self, cr, uid, context):
+        website_id = context['website_id']
+        if website_id:
+            return self.pool['website'].browse(cr, uid, website_id, context=context).default_lang_code
+        else:
+            return super(view, self).get_default_lang_code(cr, uid, context=context)