Sometimes some users complain that language of CBB app is changed to Simplified Chinese or Traditional Chinese. The reason is related to preferred language settings on your device.
For example, your order of preferred language is Italian, Simplified Chinese, English, Traditional Chinese.
Creative Book Builder currently supports English, French, Simplified Chinese, and Traditional Chinese. No Italian is provided. However, your second language is Simplified Chinese, and then iOS chooses Simplified Chinese. Therefore, language of CBB app is changed to Simplified Chinese. For more information, check this out: https://developer.apple.com/library/content/qa/qa1828/_index.html.
The solution is to change ordering of preferred language. You can just move English to be second language or move English before French, Simplified Chinese or Traditional Chinese.