This is mobile Web architecture development, not App development.
Currently a basic index.html page is presented on connection to a Wifi SSID, because web access is unavailable the mobile assumes a portal login is required. The platform is Raspberry Pi 3 B, Raspbian v8 (kernel 4.4.13-v7+), Nginx v1.6.2, Hostapd v2.3, DNSmasq v2.72. A working image can be supplied.
The solution is for an initial connection screen at the portal login page, in the language the mobile is using, which launches the on-board browser at a specified language specific URL.
FYI. See upwork job relating to an Android device issue in this area: https://www.upwork.com/jobs/Android-Launch-Chrome-window-from-the-Captive-Portal-Browser_~01ce8ec277918f4428.
Output required in English is:
Step by step additional component installation, if any.
Well commented HTML and script example files with supporting file structure.
Proof both Android and iOS mobiles are compatible with the solution.
Note: power for the Raspberry Pi in this implementation is limited, therefore a balance between ease of administration of the HTML / script solution and any additional components is required.