First check the Language Pack Directory in Host Settings.
If there is not already an LP for Forums available in your language, log into your portal as host user and go to Languages in Host Settings.
Enter the language editor, select LocalResources > DesktopModules > Forums from the tree on the left, select your language and translate one file after the other
after you finished, use "Create Language pack" to export your result and to distribute it to other DNN installations. Submit it to the Language Pack directory, so other user can download it from here.