Thank you very much for your reply and help!
I have now changed the directory and disabled cache, but still experience problems.
For example: if I go to Wishlist and sent an e-mail (Share Wishlist), I get the following message: “Your Wishlist has been shared.”
In Mage_Wishlist.csv I have added the following line:
Your Wishlist has been shared,"Je verlanglijstje is verstuurd”
Saved in directory => .../app/design/frontend/default/ma_bookstore/locale/nl_NL/
Why is the translation not recognized by Magento?
And where are the inline translations saved, ‘cause the modified dates of the Mage_xxx files never change?