Select category, which file?

Hello, sorry for my bad englisch.

I have a problem on my page http://www.foerdeaktuell.de/kleinanzeigen/anzeige-aufgeben/ .

I want to translate the passage “Select Category” to german and “select a category” in the Dropdown menu too.
I use a german language file, but there aren´t these passages.

In which file can I change these ? Or does any one know how i solve my problem?

Thanks for support.

Jakob

https://wordpress.org/plugins/another-wordpress-classifieds-plugin/