Double Installation?!?

I use WP 3.42. When new updates are available WP tells me, that I have installed 3.42 EN *and* 3.42 DE for the German Version. I can only remember do have installed 3.42 DE.

However, the site is also slightly mixed in German and US terms. The backoffice is in German, on the site (for example in the area comments) there are English terms. Just have a look at https://www.knuts-atlantik-tour.de…

Any comments about that?
What has to be done to get everything in German?
Do I have to remove one version, if yes, how?
Will the database and its tables be effected, so is there a risk to have the content get lost?

I appreciate any help.

Regards,
LiveKnut