Show form errors in corresponding translation

I am using the Uniform plugin for handling all the form stuff one needs. Our site is fully in german, so the Kirby’s config.php has the setting 'locale' => 'de_DE'. We don’t use the languages feature as we don’t expect to have multiple translations for our content.

When I’m now implementing e.g. the calc guard the error label is in English. I can see that mzur has translated alle the strings to German and several other languages. How can I force the plugin, to return the correct translation?

I’m really sure I’m missing something here… can somebody help me out? Thanks!