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I'll use the core language pack as an example. I know you're Dutch, so we'll use that.
If you unpack the DUtch language pack for XOOPS 2.0.5, you'll see that it has a few folders that correspond with existing folders in your XOOPS site. For instance, it has, at the top level, a languages/ and a modules/ folder.
The languages/ folder contains a folder called nederlands/.
The modules/ folder contain subfolders like news/language/nederlands/, newbb/language/nederlands/ etc.
It's these nederlands/ folders that contain the language specific definitions XOOPS uses.
Now, how to install:
Upload the contents from the top level of the language pack (where you see the languages/ and modules/ folders) to your webiste's root dir (the directory that also has the languages/ and modules/ folder, as well as the mainfile.php file). The structure of the language pack itself ensures that all the core modules as well as the system language is added to the site.
When you've uploaded these folders, log in to your site as administrator, go to system admin ->preferences -> general settings, and then you'll see there's a 'nederlands' option in the 'select your site's language' setting. Select and click 'go' at the bottom to confirm your changes.
Now your site is in Dutch.
This is for core modules only. For every 3rd party module you install you'll need to find or create and add a 'nederlands' folder to its 'language' folder (for example, modules/wfchannel/language/nederlands/), containing the definition files with the Dutch translation. You can do the translation yourself very easily, by copying the english language folder and renaming it to 'nederlands', and then edit the definition files in your new 'nederlands' folder.
Hope this helps!
Herko