If setting a system language in iOS doesn't take effect for apps already installed. What else can be done? Perhaps, changing the base system's language isn't the way to getting all the apps' language changed the same way.
The case is, an iPad set from factory to Chinese language. In accordance, all apps, including system as user's ones, are set to Chinese.
Despite Apple world's cultural concept that each device belongs exclusively on one person, the iPad was given away. 
The new owner is a Spanish speaker who doesn't know a little of Chinese.
Changing the system language to Spanish didn't work out most of the apps retaining Chinese. The language change is only effective to the menues and widgets of iOS itself.
While a clean reinstall does the job for most of apps, the problem stems from the system graded apps, those that being still in Chinese, don't admit a workarround like that, as they cannot be uninstalled.
So, whats the correct (or incorrect) way to go to achieve a language change this complete as though the full system were set Spanish from factory?
All apps provided by Apple should automatically change language to whatever the OS is set to.  If they do not, you should try Restart or Force Restart.

These is no further os change you can make, so if Applications you loaded don’t respect the system language, the solution is removing them or contacting their authors for a bug fix or app specific support.