In muliti-language environment with 2 or more supported portal languages the language code is deciding which KA will be shown.

If de-DE is changed to de what is recommended, no German article will be found. The same effect is English, changing the code from en-US to en

It seems a hardcoded relation between language code and KA language existing.

Please improve this behavior by add a new field with KA language in Portal Language definition or define a own search view for each language so it would be more flexible to set up the query.
In German: all German and English articles are shown
In English: only English articles are shown