Hi @devost
This is a bit challenging with the OOTB "Registration claims mapping"

The contact table has a column called "Preferred language". Unfortunately, this is a lookup, so the chances of B2C claims mapping is low.
If you are using an enhanced data model, then there is a possibility

The simple way I can think of is to trigger a Power Automate or workflow.
High-level idea
1. HTTP request received > Azure AD B2C > capture the Country input with the help of API Connectors
2. Update the dataverse > Contact table > preferred language column
Probably fun and challenging time ahead.
Hope it helps.
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