Hi
Are you using the extension in Joomla 1.6 ? In Joomla 1.6, the language file format has changed a litle and the translation function doesn't work very well (I will address this issue in the next release which I will work on next week). For now :
1. Please re-install the extension (or get the language file from the component and re-upload it to language/en-GB folder of your site) .
2. Open the language file at language/en-GB/en-GB.com_dms.ini, find the language item DMS_CATEGORIES and change it to meet your need . Upload the language file to your server .
After that, the issue should be solved .
Regards,
Tuan