Overhaul setting the `org-babel-load-languages' option

README.org

@@ -2015,11 +2015,8 @@ of the [[info:org#Top][Org (info)]] manual.
 I have split the initial [[https://orgmode.org/][Org-mode]] setup over several listings.  Here, follows a
 list detailing and motivating each listing:
 1. Listing [[lst:set-org-options]] handles basic [[https://orgmode.org/][Org-mode]] options.
-2. Listing [[lst:set-org-babel-options]] handles basic [[https://orgmode.org/][Org-mode]] options specific to
-   [[info:org#Working with Source Code][work with source code (info)]] and it
-   selects the Python interpreter by means of
-   src_emacs-lisp{(choose-common-python-interpreter)} defined in listing
-   [[lst:choose-common-python-interpreter]].
+2. Listing [[lst:setup-org-babel]] handles basic [[https://orgmode.org/][Org-mode]] options specific to
+   [[info:org#Working with Source Code][work with source code (info)]].
 3. Listing [[lst:set-org-link-options]] handles [[https://orgmode.org/][Org-mode]] options specific to
    [[info:org#Hyperlinks][hyperlinks (info)]].
 4. Listing [[lst:setup-org-mode-map]] extends the =org-mode-map= with useful
@@ -2050,19 +2047,18 @@ list detailing and motivating each listing:
      (calc . t)
      (dot . ,(fboundp 'grapviz-dot-mode))
      (emacs-lisp . t)
+     ;; (eshell . t) ; bug
      (fortran . t)
      (gnuplot . ,(fboundp 'gnuplot-mode))
      (js . t)
      (latex . t)
      (lisp . t)
-     (lua . ,(fboundp 'lua-mode))
+     ;; (lua . ,(fboundp 'lua-mode)) ; bug
      (maxima . ,(fboundp 'maxima-mode))
      (org . t)
      (perl . t)
-     ;; The Python setup uses the two functions below.
-     ;; Those functions are not yet bound during bootstrap.
-     (python . ,(and (fboundp 'choose-common-python-interpreter)
-                     (fboundp 'choose-common-python-linter)))
+     ;; Postpone Python activation to prevent calling still unbound functions.
+     (python . nil)
      (shell . t))
    org-export-backends '(ascii beamer html icalendar latex odt texinfo)
    org-file-apps '((auto-mode . emacs)
@@ -2110,8 +2106,38 @@ list detailing and motivating each listing:
   (with-eval-after-load 'sly
     (setopt org-babel-lisp-eval-fn #'sly-eval)))
 
-(with-eval-after-load 'ob-python
-  (choose-common-python-interpreter 'python))
+(with-eval-after-load 'emacs
+  (defun set-org-babel-language-activity (lang active)
+    "Set the `org-babel' activity of language LANG to ACTIVE.
+
+Let `org-babel-do-load-languages' load ob-LANG when ACTIVE is t or
+unbind the `org-babel' interface functions when ACTIVE is nil."
+    (if (locate-library (format "ob-%s" lang))
+        (or (org-babel-do-load-languages
+             'org-babel-load-languages
+             (cons (cons lang active)
+                   (assq-delete-all
+                    lang (default-toplevel-value 'org-babel-load-languages))))
+            org-babel-load-languages)
+      (warn "Can't locate `ob-%s', hence don't set `%s' activity" lang lang)))
+
+  (defun activate-org-babel-language (lang)
+    "Activate LANG for `org-babel'."
+    (interactive "SActivate language: ")
+    (if (set-org-babel-language-activity lang t)
+        (message "Did activate `%s' for `org-babel'" lang)
+      (message "Can't activate `%s' for `org-babel'" lang)))
+
+  (defun deactivate-org-babel-language (lang)
+    "Activate LANG for `org-babel'."
+    (interactive "SDeactivate language: ")
+    (if (set-org-babel-language-activity lang nil)
+        (message "Did deactivate `%s' for `org-babel'" lang)
+      (message "Can't deactivate `%s' for `org-babel'" lang)))
+
+  ;; Work around a "(defcustom org-babel-load-languages)" call bug.
+  (set-org-babel-language-activity 'eshell t)
+  (set-org-babel-language-activity 'ruby (fboundp 'lua-mode)))
 #+end_src
 
 #+caption[Set =org-link= options]:
@@ -4273,7 +4299,7 @@ install and use [[https://gitlab.com/andreyorst/ob-fennel][ob-fennel]] with the
   (unless (package-installed-p 'ob-fennel)
     (quelpa '(ob-fennel :repo "andreyorst/ob-fennel" :fetcher gitlab)))
   (when (ensure-package-selection 'ob-fennel)
-    (org-babel-do-load-languages 'fennel (fboundp 'fennel-mode))))
+    (set-org-babel-language-activity 'fennel (fboundp 'fennel-mode))))
 #+end_src
 
 #+caption[Fennel example]:
@@ -4426,6 +4452,8 @@ Finally, listing [[lst:flake8-nocolor][flake8-nocolor]] and [[lst:ruff-nocolor][
 (defun choose-common-python-interpreter (&optional interpreter)
   "Let `ob-python' and `python-mode' use the same Python INTERPRETER."
   (interactive)
+  (unless (featurep 'ob-python)
+    (set-org-babel-language-activity 'python (fboundp 'python-mode)))
   (let* ((prompt (format "Choose Python (%s): "
                           (bound-and-true-p python-shell-interpreter)))
          (choices '(ipython python))