Use English for verification error messages

Because the log page is more consistent in English.

xExtension-ReadingTime/i18n/en/ext.php

@@ -5,14 +5,12 @@ return array(
 		'speed' => array(
 			'label' => 'Reading speed',
 			'help' => 'Conversion factor to reading time from metrics',
-			'invalid' => 'Reading speed must be greater than 0',
 		),
 		'metrics' => array(
 			'label' => 'Source metrics',
 			'help' => 'Source of reading time calculation',
 			'words' => 'Number of words',
 			'letters' => 'Number of letters',
-			'invalid' => 'Unsupported source metrics',
 		),
 	),
 );