The iPhone warranty is only valid in the country in which the iPhone was purchased.
Try a reboot by holding both the power and home buttons until the iPhone shows the apple logo and restarts, ignoring the red slider if it appears.
Make a current backup of the iPhone contents, and then restore to factory conditions and then restore from the backup...some users have found that to fix the WiFi problems.
The iPhone warranty is not international. Apple will only honor the warranty in the country where the iPhone was sold.
If your WiFi connections button in Settings is greyed out, see http://support.apple.com/kb/TS1559 and if you want to Troubleshoot, and can access the router, http://support.apple.com/kb/TS1398 those may help but you probably need to have the iPhone tested by an Apple technician.