Hello Joren de Koning,
You can search in this 2 language file from your site.
JoomlaFolder\language\xx-XX\xx-XX.com_easysocial.ini
JoomlaFolder\language\xx-XX\xx-XX.com_users.ini
Or you can go to your Joomla backend > Extension > Language manager > Click "NEW" > search the word which you would like to change to > then check my screenshot :
http://screencast.com/t/jjJsNvUZJ (there have the instruction on first step)
Hope this help.