[FIX] website: set correct value in frontend_lang cookie

Before this commit, the `url_code` field value was stored in the cookie instead
of the `code` field.
This was making the language selector in the frontend to not change the lang
correctly when trying to access the website default lang, ONLY for the lang
having an `url_code` different than its `code` value (typically the case for
principal languages).

Step to reproduce:
  - Add `fi_FI` as main lang on website
  - Visit frontend and change lang to `Suomi`, it won't change

This behavior was only related to the language switcher. It did not affect
dispatching (if correct lang in url) and nearest lang finder.

Fixes #41522

closes odoo/odoo#41728

X-original-commit: b62b2828552abb38c8adc74c69cb427c3fd98605
Signed-off-by: Romain Derie (rde) <rde@odoo.com>

addons/website/controllers/main.py

@@ -113,11 +113,13 @@ class Website(Home):
 
     @http.route('/website/lang/<lang>', type='http', auth="public", website=True, multilang=False)
     def change_lang(self, lang, r='/', **kwargs):
+        """ :param lang: supposed to be value of `url_code` field """
         if lang == 'default':
             lang = request.website.default_lang_id.url_code
             r = '/%s%s' % (lang, r or '/')
         redirect = werkzeug.utils.redirect(r or ('/%s' % lang), 303)
-        redirect.set_cookie('frontend_lang', lang)
+        lang_code = request.env['res.lang']._lang_get_code(lang)
+        redirect.set_cookie('frontend_lang', lang_code)
         return redirect
 
     @http.route(['/website/country_infos/<model("res.country"):country>'], type='json', auth="public", methods=['POST'], website=True)