Every app need a default language. Due to the effect of localization, we may not expect that every app developed will be in English. For the user agents and assistive technologies to render the right content, ensure that the language is set for the project. Sometimes the default will be set and the developers have to change the defaults.
The basic setting of language of the app is set through General > Language & Region of the Settings application), however there are many ways in which the language may be predefined. Have a look at the Q & A How does iOS determine the language for my app? For more information.
In Android the default language is set to US English. If the content of the app is in a different language, developers have to make the necessary changes in the resources. The resolution in supporting multilingual content may be different in different versions. For a specific solution look at the Android developer guide in setting multilingual resources.
WCAG Success criteria
3.1.1 Language of page(Level A)