My original posting included the tex file generated by C-c C-e l l. Perusing it will show you that babel is not used anywhere in this export.
On the (remote) chance that my problem might be caused by my use of emacs 25.2, I just installe emacs 26 from the emacs-snapshot packages for Debian testing, ang got exactlu the same prolem.
The org-relevant part of my .emacs is as follows :
(require 'org)
(require 'org-ref)
Then, later
;; Septembre 2018 : org-mode
;; touches globales : À revoir ?
(global-set-key "\C-cl" 'org-store-link)
(global-set-key "\C-ca" 'org-agenda)
(global-set-key "\C-cc" 'org-capture)
(global-set-key "\C-cb" 'org-switchb)
;; Chemin de la bibliothèque ditaa
(setq org-ditaa-jar-path "/usr/share/ditaa/ditaa.jar")
;; Autoriser les évaluations Babel
(setq org-confirm-babel-evaluate nil)
;; Langages connus
(org-babel-do-load-languages
'org-babel-load-languages
'((ditaa . t)
(emacs-lisp . t)
(R . t)
(python . t)
(sagemath . t)
(julia . t)
(gnuplot . t)
(maxima . t)
(stan . t)
(mathematica . t)
(sqlite . t)
;; (sh . t)
))
Like it or not, believe it or not, this setup is sufficient to create the problem I reported.
HTH,
--
Emmanuel Charpentier