[FIX] web: do not use default en_US lang when loading translations from session

When loading translations from Session, if no lang is defined in context or html,
it is loading translations of en_US lang by default.
No default should be used to fallback on the language of the current user.

opw-2501708

closes odoo/odoo#69336

X-original-commit: 0b9ab4efcf66d4c5099a9f33d2fb1b4dd814eb5c
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>
Signed-off-by: Anh Thao PHAM <kitan191@users.noreply.github.com>

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

@@ -195,13 +195,16 @@ var Session = core.Class.extend(mixins.EventDispatcherMixin, {
         });
     },
     load_translations: function (modules=null) {
+        var lang = this.user_context.lang
         /* We need to get the website lang at this level.
            The only way is to get it is to take the HTML tag lang
            Without it, we will always send undefined if there is no lang
            in the user_context. */
         var html = document.documentElement,
-            htmlLang = (html.getAttribute('lang') || 'en_US').replace('-', '_'),
-            lang = this.user_context.lang || htmlLang;
+            htmlLang = html.getAttribute('lang');
+        if (!this.user_context.lang && htmlLang) {
+            lang = htmlLang.replace('-', '_');
+        }
 
         return _t.database.load_translations(this, modules, lang, this.translationURL);
     },