Smartphones settings allow you to choose a default language thereby ensuring your application displays in that language. Is it useful to provide an option to change this language within my application itself?
It could be useful in the following circumstances:
So yes, being able to change the language of a single application could be useful.
Having the option to switch the interface language could be very useful, if you want to let users override the system language. It depends on the type of application you're developing. If you don't provide a setting you could still switch language according to systems language. So if the user switches language from Spanish to English in the system, you're application does the same.