Use the right PHP function

src/Translatable.php

@@ -131,7 +131,7 @@ class Translatable {
 		$results = [];
 
 		foreach ( $this->languages as $language ) {
-			if ( !in_array( $language, $this->translations, true ) ) {
+			if ( !array_key_exists( $language, $this->translations ) ) {
 				continue;
 			}