I've installed a Craft Pro site with 3 locales: Dutch, French & English, but the people who are going to input data will speak those languages too, so I need to translate the instructions field in the control panel (and, if possible, the field names too since most of the time that makes it clear enough). Is this possible, and if it is: how? Thanks!
If your main language site is dutch for example, you can do that using your own php files locale and naming them according the other locales languages, en.php, fr.php and put those files in craft/translations/ (a folder you must create yourself). Then you can translate in the array all your string (field name and instructions).
Have a check at the documentation: