Mangled Belgian language (7)
I believe the answer is:
bengali
'language' is the definition.
(thesaurus)
'mangled belgian' is the wordplay.
'mangled' is an anagram indicator.
'belgian' is an anagram of 'BENGALI'.
(Other definitions for bengali that I've seen before include "Language of Bangladesh and northeast India" , "may be Indian" , "Native of Calcutta region" , "From Dhaka or Kolkata, perhaps" , "foreign language" .)