TestXHTML: install a localization business template

To reveal invalid html in erp5_web's language selector

product/ERP5/tests/testXHTML.py

@@ -151,7 +151,10 @@ class TestXHTML(ERP5TypeTestCase):
       'erp5_web_ung_theme',
       'erp5_web_ung_role',
       'erp5_ui_test',
-      'erp5_web_ung_ui_test'
+      'erp5_web_ung_ui_test',
+      'erp5_l10n_fr', # install at least one localization business template
+                      # because some language switching widgets are only
+                      # present if there is more than one available language.
     )
 
   def afterSetUp(self):