Welcome, I am Cory.
I apologize for this issue.
Please follow the steps below:
Tap Menu, it looks like 3 lines at the top right.
Tap Settings (It is the 5th option down.)
Tap the arrow next to Device Options (It is the 4th one down.)
Tap the arrow next to Personalize Your Kindle (It's the 4th one down.)
Tap the arrow next to Language (It is the 1st one on the list)
Tap English (It is the 3rd one on the list.)
I hope this helps you get the Kindle back to English. If you have anymore questions, please reply and I will assist you.
If it is fixed, please do not forget to rate my answer 3 stars or higher at the top of the page.