As mentioned you could use the maxmind db but you would need to either include the csv (both ipv4 and ipv6) in your app or create a web app that will serve you the results (ie post the ip to a php script on your server, do the lookup and return the string). But you should know that you cant just lookup the IP you will get from the device, you need to make sure
a) you will get the correct one, the phone will have an IP in a private range most probably even on gsm so nothing too lookup there, you will need again to depend on an external service (lots of those available and ofc its easy to just have a php script to give you the IP of the phone on a simple post to it)
b) the max mind DB's include IP ranges, not a full list of IP's ie 192.168.1.0 - 192.168.1.255 if you are lucky or a class like 192.168.1.0/24 which makes queries even more difficult
I would just propose you just check the phone locale, many users prefer to have the language they have selected on the phone, I am greek but I still have my phone on the en.US locale cause I prefer it.