Dashboard language mixed up

I have added this line to my functions.php:
add_filter('gettext', 'theme_filter_text', 10, 3);
(followed by the rest of the definitions, and it all does the trick as intended).
But by adding this line my Dashboard switches to English, even though I have installed the Dutch version of WP. The moment I remove that line it’s back to Dutch again, but obviously I need this line of code.

Any ideas how to get back my default language?
TIA, Leo