I am not sure if this will work if you are already on global rom but it should work as well.
DO THIS AT YOUR OWN RISK!!! I am not liable to any damage if you accidentally brick your device.
Steps on upgrading from CN Rom to Complete Global Rom
(Check your ROM, if you are on 1907 firmware, skip to step 3)
1) Download the WW International rom from here
2) Extract the file on your PC. Boot the phone into fastboot mode (hold volume up then press and hold power button) connect to the side port and run "flashall_AFT.cmd" wait for it to finish and the phone will restart itself.
3) Download the ROG Phone Unlock Tool and copy it on your phone and install it then run it. The phone will boot twice. DO NOT PRESS POWER TO STOP THE BOOT. It will wipe your data automatically.
4) Download and install Minimal ADB and Fastboot app (windows). Install it on your PC.
5) Download the rootww190733.img and save it to the Miniman ADB and Fastboot Folder (C:\Program Files (x86) \Minimal ADB and Fastboot)
6) Run Minimal ADB and Fastboot. Switch the phone into fastboot mode (hold volume up then press and hold power button) then type "fastboot flash boot rootww190733.img"
7) Reboot the phone and connect it to PC, copy and install Magisk and Root Explorer
https: //tinyurl.com/yxe66uol - Root Explorer
https: //tinyurl.com/y6yr3sem - Magisk
-run magisk, update and grant access to root explorer
8) Open the Root Explorer App on you phone. Then from the app, select Mount R/W to R/O. Search for "/vendor/factory" folder and Edit the file content COUNTRY from CN to WW (ALL Capital). Scroll to "/system/etc/AsusTheme/themes" then delete the hidden file to enable the hidden themes.
9) Restart the Phone. You are done with the first step.
The next part will unroot your phone and relock the bootloader.
1) Download the stock boot loader "bootstockww190733.img and save it to the Miniman ADB and Fastboot Folder (C:\Program Files (x86) \Minimal ADB and Fastboot)
2) Open Minimal ADB and Fastboot, switch your phone into fastboot mode, then connect to PC and type "fastboot flash boot bootstockww190733.img"
- after flashing, you will need to wipe/reset and boot back to fastboot mode
3) While on fastboot mode and ADB, Relock the bootloader and run "fastboot oem asus-csc_lk"
- this will relock your bootloader
4) After relocking the bootloader it will erase everything up automatically. If not, boot to recovery mode and wipe/reset. You are done!