fix #4410 The string to escape is not a valid UTF-8 string (#10050)

plugins/SegmentEditor/SegmentFormatter.php

@@ -9,6 +9,7 @@
 namespace Piwik\Plugins\SegmentEditor;
 
 use Exception;
+use Piwik\Common;
 use Piwik\Config;
 use Piwik\Db;
 use Piwik\Piwik;
@@ -111,7 +112,7 @@ class SegmentFormatter
             $translation = Piwik::translate($this->matchesMetric[$operator]);
         }
 
-        return strtolower($translation);
+        return Common::mb_strtolower($translation);
     }
 
     private function getFormattedValue($operand)

plugins/SegmentEditor/SegmentSelectorControl.php

@@ -53,7 +53,7 @@ class SegmentSelectorControl extends UIControl
                 && ($segment['type'] == 'metric' && $segment['segment'] != 'visitIp')
             ) {
                 $metricsLabel = Piwik::translate('General_Metrics');
-                $metricsLabel[0] = strtolower($metricsLabel[0]);
+                $metricsLabel[0] = Common::mb_strtolower($metricsLabel[0]);
                 $segment['category'] .= ' (' . $metricsLabel . ')';
             }
             $segmentsByCategory[$segment['category']][] = $segment;