In the last post I discussed why internationalization was important, but the follow on from this is when should a plugin be internationalized.
When developing a plugin, it is tempting to work on the functionality first and then go back to internationize the strings afterwards. When I started developing plugins, none of them were interationalized. When I started doing so, I had to go back and find all the strings and then wrap them in the relevant functions.
New plugins, I have internationalized from the beginning and this is much less work and also means there is much less opportunity to miss a string which going back afterwards makes quite likely.
My recommendation, is to internationalize your plugin from the beginning and thereby save yourself time and errors.