Since he translated this language (7)
I believe the answer is:
chinese
'this language' is the definition.
(Chinese is a language)
'since he translated' is the wordplay.
'translated' indicates anagramming the letters.
'since'+'he'='sincehe'
'sincehe' with letters rearranged gives 'CHINESE'.
(Other definitions for chinese that I've seen before include "Since he is from a very populated nation" , "Language spoken by many" , "Member of Han people" , "- Chequers; - leaves; - puzzle" , "CIE hens from Beijing, say" .)