Setting org-babel-load-languages in .dir-locals.el

Question

As per the title, I'd like to set the org-babel-load-languages (to contain ditaa and calc languages) for a specific project only.

The only solution I found so far is this one

(
 (org-mode .
           ((eval . (org-babel-do-load-languages
            'org-babel-load-languages
            '((emacs-lisp . nil)
              (calc . t)
              (ditaa . t)))))))

The main drawback is that in this way the variable changes its value globally, while I'd like to have the different value only for the project.

So, I tried using

(
 (org-mode .
           (
            (org-babel-load-languages . ((ditaa . t) (calc . t)))
            )))

and a lot of different flavors of it, without any success.

So, is it possible to assign this variable on a per-project basis?

Notes

The variable documentations says the following after using the second method

org-babel-load-languages is a variable defined in ‘org.el’. Its value is ((ditaa . t) (calc . t)) Original value was ((emacs-lisp . t))

Local in buffer LAB.org; global value is ((python . t) (shell . t) (emacs-lisp . t))

This variable’s value is directory-local, ...

which seems fine, but still org-mode emacs is unable to execute functions for calc or ditaa.

The first method (with eval), instead, works fine.