That will depend on the application in question and how much of the code is directly related to the GUI.
All of the business logic , AJAX requests etc should be fairly reusable.
I know there have been plenty of times building web apps where I have had to maintain separate front-end and back-end code in different languages that does exactly the same thing.
Time can also be a factor, if you are already maintaining a web client , a mac client in obj-C and possibly an android Java client having to learn yet another language and ecosystem is probably not what you want.
All of the business logic , AJAX requests etc should be fairly reusable.
I know there have been plenty of times building web apps where I have had to maintain separate front-end and back-end code in different languages that does exactly the same thing.
Time can also be a factor, if you are already maintaining a web client , a mac client in obj-C and possibly an android Java client having to learn yet another language and ecosystem is probably not what you want.