Tools for the translator
Since the translating that needs to be done is in the form of webpages, you will need to use a HTML editor to edit the files.
HTML editor
A really good open source HTML editor is Komposer.
It can be downloaded from here: http://www.kompozer.net/download.php. Most people will use the 32 bit version.
For Windows:
Kompozer doesn't need to be installed. Simply extract the zip file you downloaded and start the program by doubleclicking the Kompozer.exe file in the extracted folder.
For Linux:
You can probably get it through your packagemanager by searching for kompozer. Otherwise download it from the website like everybody else.