Language files are overridden with update

Hi,

I was following the approach described by the-events-calendar (https://theeventscalendar.com/knowledgebase/how-to-override-plugin-language-files/) in order to adapt the standard German language files to my needs. However, wordpress keeps updating my language files so that I always see the old text again after a while. Any hints what I might do wrong?

Cheers,
Magamir.