Hi Nico,
Did you make the changes using the Translation Utility? If so, there are extra steps to get your changes in effect.
Using the Translation Utility is the first step. This creates a new file in /system/expressionengine/translations/ That file then has to be placed in a folder named for the language you have translated into. That language then needs to be moved into /system/expressionengine/languages/ and the new language becomes available as a Language choice in the Control Panel.
If you only want to change this one file, you can simply replace the file in the english language folder with your translation and it will be available when selecting English as the language.
Let me know if you have any other questions!
Cheers,