#+LANGUAGE: fr

(defconst org-export-dictionary
    ...
    ("Table of Contents"
         ...
         ("sr" :html "Sadržaj" :utf-8 "Sadržaj")
         ...)
    ...)

(defun org-export-translate-to-lang (term-translations &optional lang)
  "Adds desired translations to `org-export-dictionary'.
   TERM-TRANSLATIONS is alist consisted of term you want to translate
   and its corresponding translation, first as :default then as :html and
   :utf-8. LANG is language you want to translate to."
  (dolist (term-translation term-translations)
    (let* ((term (car term-translation))
           (translation-default (nth 1 term-translation))
           (translation-html (nth 2 term-translation))
           (translation-utf-8 (nth 3 term-translation))
           (term-list (assoc term org-export-dictionary))
           (term-langs (cdr term-list)))
      (setcdr term-list (append term-langs
                                (list
                                 (list lang
                                       :default translation-default
                                       :html translation-html
                                       :utf-8 translation-utf-8)))))))

(org-export-translate-to-lang '(("Table of Contents"
                                 "Sadržaj"
                                 "Sadržaj"
                                 "Sadržaj")
                                ("Another term"
                                 "coilogji"))
                              "sr")

#+LANGUAGE: fr

#+LANGUAGE: fr
#+LATEX_HEADER: usepackage[AUTO]{babel}

#+LANGUAGE: br
#+LATEX_HEADER: usepackage[breton]{babel}

#+LANGUAGE: en
#+LATEX_HEADER: usepackage[AUTO]{babel}
#+LATEX_HEADER: addtocaptionsenglish{renewcommandcontentsname{Outline}}

#+LATEX_HEADER: renewcommand*{contentsname}{My Table of Contents Header}

Symbol's value as variable is void: org-export-dictionary