After I once changed the language to Thai and then back to English, my app store remains in Thai and I can't change it back... Signing out/in didn't work... help!?

Have you done the following?

--> Go to "setting" from homescreen in ipad
--> select "International" in "General" setting
--> tap on "language"
--> you will see different languages. select "English" as your default language.
--> Open the application store. you will see application language english.

Go into the AppStore then click on your Apple ID then View apple ID then change your country/region

R.I.P. Justsumhelp