Ignore non-existing translations

src/Translatable.php

@@ -131,6 +131,10 @@ class Translatable {
 		$results = [];
 
 		foreach ( $this->languages as $language ) {
+			if ( !in_array( $language, $this->languages, true ) ) {
+				continue;
+			}
+
 			$result = $this->installTranslation( $this->translations[ $language ][0]->package );
 
 			if ( $result ) {