I also had the same issue in AMOS for the es_mx language pack for a while. Eventually, I uploaded a php file from a language customization and AMOS started working again.
General developer forum
AMOS is not saving the strings
Αυτή η συζήτηση κλειδώθηκε κι έτσι δεν μπορείτε να απαντήσετε σε αυτή.