Index: openacs-4/packages/acs-lang/tcl/locale-procs.tcl
===================================================================
RCS file: /usr/local/cvsroot/openacs-4/packages/acs-lang/tcl/locale-procs.tcl,v
diff -u -N -r1.50 -r1.51
--- openacs-4/packages/acs-lang/tcl/locale-procs.tcl	25 Jul 2018 19:37:26 -0000	1.50
+++ openacs-4/packages/acs-lang/tcl/locale-procs.tcl	6 Nov 2018 10:41:15 -0000	1.51
@@ -658,7 +658,11 @@
         if {[lang::conn::valid_locale_p $elm]} {
             lappend acclangv $elm
         } else {
-            error "invalid locale in provided Accept-Language header field"
+            # It is usually bots or other kinds of not-canonical web
+            # browsers which set this wrong. We tolerate it by
+            # assuming our default language.
+            ns_log warning "Invalid locale '$elm' in provided Accept-Language header field. Defaulting to system locale."
+            return [lang::system::locale]
         }
     }