Two promising words in contrived language (3)
I believe the answer is:
ido
I believe this clue is a double definition.
'two promising words' is the first definition.
('I do' from marriage vows)
'contrived language' is the second definition.
(invented language based on Esperanto)
'in' is the link.
(Other definitions for ido that I've seen before include "Marriage response" , "Prepared tongue" , "International language" , "See 1" , "Wedding vow" .)