1. Enable localization as explained here: Localization and create a new language from Administration. Make sure that locale is the same as the locale of your language pack (for instance, fr_FR.UTF-8).

2. You'll notice that activeCollab created a folder for your translation files in /activecollab/localization folder (something like /activecollab/localization/fr_FR.UTF-8). Place translation files from language pack in that folder and see if they are available on language details in administration.

That should be it.

Is /activecollab/localization folder writable? It needs to be in order for activeCollab to create a folder for your translation for you. If not you'll need to create that folder manually. Make sure that folder name is exactly the same as locale for that language.

In that case you'll need to contact your hosting provider to chmod file for you or give you FTP user with proper permissions.

It's not common, but in some cases system administrators configure system so files created by PHP cannot be altered by FTP users...

These files are created by PHP so they get permissions PHP process gives them. This cannot be controlled from within activeCollab and it specific to your servers setup.

If you are server administrator you should be able to change permissions and edit that file.