Hi macbahi,
Yes there is actually a bug in PLUS+1 GUIDE 12.1.8 that change the language "by itself". If you update your GUIDE version to 12.2.9 you should not have this problem.
In version 12.1.8 it's pretty easy to get back to English. There is a drop down list in the toolbar to change the language. It's hard to know if you don't know Chinese.
I have posted two images below that should explain it. I hope it helps.
Have a nice day and stay safe.