[O] org-mode pcomplete does not complete added babel extra languages

[stardiviner]

 For example, I have a package =ob-restclient= installed.

 I type ~<s~ then press =[Tab]= to expand.

 #+begin_src |

 #+end_src

 I type ~rest~ at "|" position.

 But org-mode's =pcomplete= does not complete those added extra babel
languages. (I
 use ~company-mode~'s ~company-capf~ for Org-mode's ~pcomplete~.)

 In other way: I press =[C-c C-v C-d]= to insert a babel src block, it has
 ~restclient~ as option.