How to set the default internationalization language in iOS9

Question

That is, the APP supports two languages, one is Simplified Chinese and the other is English. If the system language is Simplified Chinese, then the APP is in Simplified Chinese. If the system language is English, then the APP is in English. If the system language is a third-party language, such as Japanese. So how does he obtain his language at this time, and what is the default language?