Hi Clara,
Thanks for your note. I looked at my test site and the translation appears to work properly there. Can you confirm that you are running the latest versions of WordPress (4.1.1) and The Events Calendar (3.9.1).
If you are running the latest versions and still seeing that issue, it would be worth testing for a theme conflict. It’s possible that something about your theme is messing up that one string. In that case, you might try getting in touch with your theme developer to see if they can help.
Another option is to try is using a plugin like Say What? to directly target that string and have it translated.
Best,
Leah