My problem is somewhat similar:
I've bought Kindle Fire HD 7" in Germany and Kindle Fire HD 8.9" in the US.
The German one, being available globally, freely can be set to whatever country I want. I just have to change the country setting in my Amazon account.
The US one only can be set to the US, whatever country I choose in the Amazon settings.
Simple explanation from Amazon: Kindle Fire HD 8.9" isn't available regularly in Germany, so some features won't work here.
To have both Fires in sync, I've set both to the US account.
But: I switch to the respective accounts via country settings in the Amazon account, not by changing the language on the Kindle itself. I couldn't switch to German on the US only Fire HD 8.9" anyway...
For my Kindle Paperwhite, bought in Germany, I freely can switch back and forth between the US and Germany.
But: In my Amazon account, I've got the respective billing addresses for the US and Germany.