Hi VDJ SAS,

Here you can do this by

• creating new field or

• adding context(It will display city, state, etc too)

Like 

1) By new field(Create new field in sale.order in your custom module)

    related_partner_country = fields.Char("Customer Country", related="partner_id.country_id.name")

2) By adding context in view

    <field name="partner_id" domain="[('customer','=',True)]" context="{'search_default_customer':1,'show_address': 1}" options="{&quot;always_reload&quot;: True}"/>

Hope this will help you...

Thanks!