Changing the language of the user interface can be useful e.g. if you want to test your new translation.

Usually, Hugin will use the current locale to determine the language of buttons, menus etc... Set the Language if you need to switch languages temporarily or if you are using a platform such as Windows95 that doesn't support localised software. Hugin won't change language immediately, you will need to stop and restart it.

The "language" option in the Hugin Preferences doesn't work for the Mac version. On Mac OS X the system's country settings will be used instead: change the language setting there (i.e. drag the preferred language to the top of the list), and Hugin will reflect that when you restart the program.
An alternative but more time consuming way to set another language than the current system language is to quit Hugin, reveal it's icon in the Finder, click on it and open the Information window (Cmd.-I). After deselecting all unwanted language options you can start Hugin in your preferred language.