*Org Src mrb.el

;; Register a new input method
(register-input-method
 "MyTeX" "UTF-8" 'quail-use-package
 "\\" "Personalized TeX input method"
 "~/.emacs.d/mytex-inputmethod")
 
(defun mrb/set-input-method()
  (interactive)
  (setq default-input-method "MyTeX")
  ;; Toggle only if it not active yet
  (if (not current-input-method)
      (toggle-input-method)))

(add-hook 'set-language-environment-hook 'mrb/set-input-method)
;; And now we can set it
(set-language-environment "UTF-8")

;; So we have it active at this point, but for new buffers it's still
;; not active, let's do that for the most common modes I use
;; FIXME: this works ok-ish, but is not a very clean solution I think.
(add-hook 'text-mode-hook 'mrb/set-input-method)
;; Orgmode is derived from outline-mode -> text-mode so this should already work.