Since 5.0.0:

Sets the Wi-Fi country code or region code. The default is WLAN_CC_US. This is not automatically set; you must manually set it if the country you are using the device in requires setting a specific Wi-Fi channel plan. The setting is stored in the DCT (configration flash memory) and is not reset when flashing user firmware, Device OS, or when clearing Wi-Fi credentials.

int wlan_set_country_code(wlan_country_code_t country, void* reserved);

wlan_set_country_code(wlan_country_code_t::WLAN_CC_JP, nullptr);
Country Code Value
wlan_country_code_t::WLAN_CC_UNSET 0x0000
wlan_country_code_t::WLAN_CC_US 0x5553
wlan_country_code_t::WLAN_CC_EU 0x4555
wlan_country_code_t::WLAN_CC_JP 0x4A50
wlan_country_code_t::WLAN_CC_CA 0x4341
wlan_country_code_t::WLAN_CC_MX 0x4D58
wlan_country_code_t::WLAN_CC_GB 0x4742
wlan_country_code_t::WLAN_CC_AU 0x4155
wlan_country_code_t::WLAN_CC_KR 0x4B52
wlan_country_code_t::WLAN_CC_WORLD 0xFFFE

This API is available only on the P2, Photon 2, and M-SoM in Device OS 5.0.0 and later.

This setting is stored at the DCT offset DCT_COUNTRY_CODE_OFFSET (country_code, offset 1758, 2 bytes) and thus can also be set in DFU mode using the -a 1 to store in alt bank 1 (DCT).