Hello.

The `m-x customize’ path to `org-babel-load-languages’ or the following method won’t accept C++.

#+BEGIN_SRC elisp :results none
  (org-babel-do-load-languages
   'org-babel-load-languages
   '((C++ . t)))
#+END_SRC

The messages buffer has the following.

   executing Elisp code block...
   org-babel-do-load-languages: Cannot open load file: No such file or directory, ob-C++

M-x org-version
Org mode version 9.1.9

M-x emacs-version
GNU Emacs 26.1
rc1

Section 14.7 Languages in the Org Mode working with source code infopages lists C++.