hi Hax, thanks for your reply.

The problem persists with the latest tranlation files.

Now that you mention other plugins, I had installed (and active) this one: http://shinephp.com/plugins-language-switcher-wordpress-plugin/

It changes WordPress’ language. Now I deactivated it, so my admin is now in english again, including the image-store plugin. So this makes me think that maybe, the translation files I downloaded from this site never worked in the first place.

Can you tell me how to verify if they are installed properly?

I placed the files at wp-content/languages/_ims, is this right? Do I need to set something else or just put the files there? thanks!