If you want to help out with making GrandPerspective available in your mother tongue, or any other language you speak fluently, go to the GrandPerspective Localization forum to express your interest.
Do not start localization yet! Apple has introduced Base Internationalization as the preferred mechanism to internationalize applications. GrandPerspective does not yet support this, but I am planning to change that, so please wait for that to be done before you start your translation efforts.
Below you can find some things to watch out for:
Make sure that text files are using the right text encoding:
Do not translate the content of the AppleTitle meta tag in the main help file. The content of this tag in the GrandPerspectiveHelp.html should be set to "GrandPerspective Help", as the help documentation will otherwise not load in the Help Viewer. The reason is that the content must match the value of CFBundleHelpBookName in the application's Info.plist file (which is not localizable).