GForge is constantly developed and so translation files are regularly outdated. Thus translations should be regularly updated to be uptodate.
If you are maintaining a translation file, you may find useful language_file_merger.php
script you can find in tools module (see GForge CVS repository for more information).
You can use the following command to merge your outdated language file with Base.tab :
php -q language_file_merger.php <your/gforge/install/root> <language> \ 1>merge.tab 2>merge.log
You have to check lines with #TO_TRANSLATE# and #TO_REMOVE# flags and respectively translate them and remove them. Lines are already sorted by alphabetical order so you just have to add header information found in the previous YourLanguage.tab file to merge.tab file and replace YourLanguage.tab by merge.tab.