For IOS 13 and above, please head to your iPhone settings--CamScanner--language to select the language, and then please check in the CS app again. 
Note: CamScanner can only support Simplified Chinese, Traditional Chinese, English, Japanese, Korean, German, French, Spanish, Portuguese, Russian, Italian, Polish, Czech, Arabic, Turkish and Slovak for now.

Below IOS 13, please open your iPhone/iPad settings--general--language & region--edit--put your mother tongue on top and move English to the second place-->finish. Then CamScanner will be in English.