I18n issues still not solved

According to the source code, this string is not translatable because the gettext function shouldn’t include variables in it.

__( 'This plugin (' . $args['plugintocheck'] . ') is <strong class="gtm4wp-plugin-not-active">not active</strong>, enabling this integration could cause issues on frontend!', 'duracelltomi-google-tag-manager' );

should be changed to:

sprintf( __( "This plugin (%s) is <strong class='gtm4wp-plugin-not-active'>not active</strong>, enabling this integration could cause issues on frontend!", "duracelltomi-google-tag-manager" ), $args['plugintocheck'] );