Not the country, I'm using ES_ADDRESS-LANGU from BUPA_ADDRESS_GET_DETAIL. Then based on the type of partner that is being created (ct_bupa_header_details-type), person or org, I call the update with that language.
Person - BAPI_BUPA_CENRAL_CHANGE - centraldata-partnerlanguage and centralperson-correspondencelanguage.
Organization - BAPU_BUPA_ADDRESS_CHANGE - addressdaa-langu
Hope it helps.