How to programmatically extract configurations from a defcustom variable?

How do I programmatically extract key-value pairs from "Babel Languages" list in defcustom org-babel-load-languages section of org.el?

I want to read the "Babel Languages" configuration into an org-mode code block and return the full list of the available programming languages.

For example, I'm want to transform the input below:

(const :tag "Awk" awk)
(const :tag "C" C)
(const :tag "R" R)

into an output value like this

(("Awk") ("C") ("R"))

I included the specific defcustom section from org.el for clarification.

(defcustom org-babel-load-languages '((emacs-lisp . t))
  "Languages which can be evaluated in Org-mode buffers.
This list can be used to load support for any of the languages
below, note that each language will depend on a different set of
system executables and/or Emacs modes.  When a language is
\"loaded\", then code blocks in that language can be evaluated
with `org-babel-execute-src-block' bound by default to C-c
C-c (note the `org-babel-no-eval-on-ctrl-c-ctrl-c' variable can
be set to remove code block evaluation from the C-c C-c
keybinding.  By default only Emacs Lisp (which has no
requirements) is loaded."
  :group 'org-babel
  :set 'org-babel-do-load-languages
  :version "24.1"
  :type '(alist :tag "Babel Languages"
        :key-type
        (choice
         (const :tag "Awk" awk)
         (const :tag "C" C)
         (const :tag "R" R)
         (const :tag "Asymptote" asymptote)
         (const :tag "Calc" calc)
         (const :tag "Clojure" clojure)
         (const :tag "CSS" css)
         (const :tag "Ditaa" ditaa)
         (const :tag "Dot" dot)
         (const :tag "Emacs Lisp" emacs-lisp)
         (const :tag "Fortran" fortran)
         (const :tag "Gnuplot" gnuplot)
         (const :tag "Haskell" haskell)
         (const :tag "IO" io)
         (const :tag "Java" java)
         (const :tag "Javascript" js)
         (const :tag "LaTeX" latex)
         (const :tag "Ledger" ledger)
         (const :tag "Lilypond" lilypond)
         (const :tag "Lisp" lisp)
         (const :tag "Makefile" makefile)
         (const :tag "Maxima" maxima)
         (const :tag "Matlab" matlab)
         (const :tag "Mscgen" mscgen)
         (const :tag "Ocaml" ocaml)
         (const :tag "Octave" octave)
         (const :tag "Org" org)
         (const :tag "Perl" perl)
         (const :tag "Pico Lisp" picolisp)
         (const :tag "PlantUML" plantuml)
         (const :tag "Python" python)
         (const :tag "Ruby" ruby)
         (const :tag "Sass" sass)
         (const :tag "Scala" scala)
         (const :tag "Scheme" scheme)
         (const :tag "Screen" screen)
         (const :tag "Shell Script" sh)
         (const :tag "Shen" shen)
         (const :tag "Sql" sql)
         (const :tag "Sqlite" sqlite))
        :value-type (boolean :tag "Activate" :value t)))