OK I got it.
I added the following in the section files of my child theme.
echo __($integral[‘my_custom_string’], ‘my_custom_domain’);
my_custom_string corresponds to the ID of a text field that I’ve defined in a custom option.
Anyway, I think this is not really a solution but more a workaround… According to the WPML documentation, admin-texts could be simply defined in a wpml-config.xml.
Maybe I’m doing something wrong.