Add a missing property (#39233)

Add a parameter called "decimal" into the LanguageSettings interface. Using this documentation as backup:
https://datatables.net/examples/basic_init/comma-decimal.html

types/datatables.net/index.d.ts

@@ -1784,6 +1784,7 @@ declare namespace DataTables {
         infoEmpty?: string;
         infoFiltered?: string;
         infoPostFix?: string;
+        decimal?: string;
         thousands?: string;
         lengthMenu?: string;
         loadingRecords?: string;