Hola Amaia,
If possible, I would recommend that you try upgrading to a newer version of AtoM. It looks like we addressed indexing many of the notes fields in the 2.4.1 release. See:
We have just released version 2.7.0, see:
I am not sure if it will work, but in the meantime you could try performing an expert search on a field. For example, to search for gato blanco in the General Notes field of a Spanish (es) archival description, try searching:
- generalNotes.i18n.es.content:"gato blanco"
Good luck!