[FIX] web : translate all content in customer preview

When we go into customer preview of a quotation for example and change the language different than English (to fr_BE for example)
we see that some of the content isn't translated  and still displaying in english.

Steps to reproduce the error :
1-Install sales and website app
2-Create a quotation for a customer and just save it don't confirm it.
3-Go to customer preview
4-Change the language to fr_BE
5-Click on "signer & payer"
6-You can see that it still display 'Full Name' in english and also some other terms

The origin of the problem is that all the parts that were rendered via a t-call gets rendered in english.
I investigated the problem and it turns out that the language code for the _t is alwyas english(https://github.com/odoo/odoo/blob/06719a84c52c7a97487b7ee0bef3bbe166d5b502/addons/portal/static/src/js/portal_signature.js#L45)
The problem is that the user_context.lang is always undefined here (https://github.com/odoo/odoo/blob/06719a84c52c7a97487b7ee0bef3bbe166d5b502/addons/web/static/src/legacy/js/core/session.js#L201)
As a solution I copied the code from 15.0 which was working fine (https://github.com/odoo/odoo/blob/4fa2dbcaf777c23758519ab0feb9707a7592cd6b/addons/web/static/src/legacy/js/core/session.js#L199-L209).

opw-3192666
.

closes odoo/odoo#125638

X-original-commit: 441c94270a4935d2b0b67a9f8201fcba9d15d957
Signed-off-by: Julien Mougenot (jum) <jum@odoo.com>

addons/web/static/src/legacy/js/core/session.js

@@ -197,7 +197,13 @@ var Session = core.Class.extend(mixins.EventDispatcherMixin, {
         });
     },
     load_translations: function (modules=null) {
-        return _t.database.load_translations(this, modules, this.user_context.lang, this.translationURL);
+        var lang = this.user_context.lang
+        var html = document.documentElement
+            , htmlLang = html.getAttribute('lang');
+        if (!this.user_context.lang && htmlLang) {
+            lang = htmlLang.replace('-', '_');
+        }
+        return _t.database.load_translations(this, modules, lang, this.translationURL);
     },
     load_js: function (files) {
         var self = this;