I got solution for it.
This is not cordova or phonegap releated changes.
Since it is iOS project, so I needed to make changes in the '.plist' file of Xcode.
<key>CFBundleLocalizations</key>
<array>
<string>en</string>
<string>pt</string>
</array>
Now above changes makes application support both English and Portuguese language in cordova plugin.