Try a search like this:
frappe@ubuntu:~/frappe-bench$ find . -name ‘*.csv’ | xargs grep ‘Solicitando el pago’
./apps/erpnext/erpnext/translations/es.csv:apps/erpnext/erpnext/accounts/doctype/payment_request/payment_request.py,Requesting payment against {0} {1} for amount {2},Solicitando el pago contra {0} {1} para la cantidad {2}
This sheds light where message translation occurs in the code:
The set of csv files are just a dictionary of key, value pairs by country en, es, fr, de, where the key is the English message phrase and the value is the translated version Spanish, French, German and so on.