A customer account has different locales in POA and PBA. It may cause troubles with some of customer's services, e.g. if a customer has French language in PBA and English language in POA then APS application may be installed with the English locale (since it is configured in POA) without possibility to change it.
POA Control Panel does not allow to change locale of a customer account, only locale of a staff member may be changed.
The problem is going to be solved in the future updates to products in scope of the request PBA-42265.
In the meantime locale of an existing customer account in POA may be changed in the system database, contact Parallels Support Team in case you need to do this.
PBA does not send information about a customer locale during new account creating using the pem.addAccount POA OpenAPI method.
PBA does allow to change locale of a customer account, however, POA OpenAPI does not allow to change locale of an existing account (POA API only allows to set up locale during account creating).