Symptoms
Multi-language welcome message has been configured as indicated in the Azure CSP documentation.
The corresponding file has been created for a new language, e.g. Messages.it.resx for Italian.
A new customer is created with country set as Italy and language set as English. But the welcome message is received in Italian, whereas English is expected.
Resolution
The welcome email only takes OA account CountryName parameter as the indicator of the message language. The limitation comes from OA Account representation in APS controller.