@Pau: The form labels in backend (if you are talking about these) are added dynamically and translated in context of the WordPress ‘default’ domain.
A possible solution for you could be to use a translated version of the whole fieldgroup/fields and adjust the labels there.
There is currently no way to find these labels with an automated tool like WPML theme/plugin scanner.
More about wrong/bad textdomain in ACF here.
A solution for WPML automated scanner could be 1.) an own textdomain for these labels, e.g. acf_label and 2.) a dynamic creation of a .php file in a path where WPML theme/plugin scanner would find them which contains all the used labels in default language like e.g. $label = __('Label 1', 'acf_label') = __('Label 2', 'acf_label') = __('Label 3', 'acf_label') ...
