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:
- When the user's native language isn't English, but they are more familiar with English labels than the native equivalents.
- For users who need to switch keyboards (and auto-complete dictionary) using both their native language and English (or another secondary language).
- For users that give their old phones to their kids, and need to set the language back to native language...
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.