Applications on a Mac, including Tunnelblick, use the Mac's System Preferences settings. In the "Languages & Text" panel you drag and drop languages to put them in the order you wish. If an application does not include your top language, the next language down is used. If the application doesn't have that, the next one down after that is used, etc.
Tunnelblick is open source -- free -- and if you can help refine the translations, we would be glad to have your help. Email me privately (my Gmail address is jkbullard) for more info about helping.