clear the "what was the last language?" state when moving out of the module language

so that things get initialized properly when moving back into the module language.

closes PR 12493

collects/drracket/private/module-language-tools.rkt

@@ -140,6 +140,8 @@
             [in-module-language? 
              (move-to-new-language)]
             [else
+             (set! hash-lang-language #f)
+             (set! hash-lang-last-location #f)
              (clear-things-out)])))
       
       (define/public (move-to-new-language)