u mean system language? Just enter in the system setting, and select the language settings, then select "English".
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Worst case, if the advise given by Vyger and Verence doesn't work for you, take ti to the carrier and ask for tech support. The carrier should be able to switch it back for you without any difficulty.
Find the printed manual or look for an online version of the manual (in English of course).
Look for the way to change the language. If you are lucky, it has pictures that show the menus. From there you can count your way through the menu. E.g. main menu - 3rd option, then 5th option then 2nd option..
Btw. It would help if you mention what phone you are using.
Find someone who speaks the language it is now set with and ask them to help you to follow the menues.