fr0nk

I have this error too, only that something inside wordpress changes my display_errors setting, so I see this on my site sometimes because it’s cached.
This seems to happen when a client doesn’t set the HTTP_ACCEPT_LANGUAGE header. Probably search engines. The fix is to check that the variable exists before using it. Here’s a patch:

--- a/wp-content/plugins/wc-fields-factory/includes/wcff-locale.php	2018-04-26 11:22:44.217060270 +0200
+++ b/wp-content/plugins/wc-fields-factory/includes/wcff-locale.php	2018-04-26 11:26:04.829774539 +0200
@@ -206,10 +206,10 @@
     public function detrmine_current_locale() {
     	$locale = "en";
     	/* Locale from Browser */
-    	if(function_exists("locale_accept_from_http")) {
-    		$locale = locale_accept_from_http($_SERVER['HTTP_ACCEPT_LANGUAGE']);
-    	} else {
-    		if(isset($_SERVER['HTTP_ACCEPT_LANGUAGE'])) {
+	if(isset($_SERVER['HTTP_ACCEPT_LANGUAGE'])) {
+	    	if(function_exists("locale_accept_from_http")) {
+	    		$locale = locale_accept_from_http($_SERVER['HTTP_ACCEPT_LANGUAGE']);
+    		} else {
     			$locale = $_SERVER['HTTP_ACCEPT_LANGUAGE'];
     		}
     	}