If you only develop your Facebook canvas application in your own local language, then you should try making your app to support other foreign languages. Facebook is currently available in over 70 languages and by supporting multiple languages will make you app more rival.
The easiest way of doing so is of course setting up separated FB app for each language. However, this is definitely not the best way. A better approach is to render the display based on the user locale / language preference.
It is good that Facebook does help us on this. When FB users access your canvas application, Facebook will include the user locale information in the signed request. With this, we don’t need to bother methods like geo-IP detection, lang preference based on the browser info etc.
For details, see the Facebook documentation at http://developers.facebook.com/docs/authentication/signed_request/.