Your template obviously has K2 overrides that are missing the media playback feature and possibly overwrite the date format.

This is easy to spot. Just load your site with the Protostar template (in which case, K2 loads its default template overrides): www.kolding-netavis.dk/dr-saetter-fokus-pa-genforeningens-100-arsdag?template=protostar

The video plays just fine as you can see.

The date format is still the same though, which indicates you probably didn't change the right language string for it.

To find out the language string that's used for your template, simply open the item.php file and search for:
...to see all available instances of date formattings.

The default template e.g. for item.php uses K2_DATE_FORMAT_LC2 as you can also see here: github.com/getk2/k2/blob/master/components/com_k2/templates/default/item.php#L36

Your language pack for Danish *may* be broken. Unzip the file and search for this exact string \\"_QQ_" in all files. If it exists, mass-replace with: \"

\"_QQ_" breaks parsing of the language files and as such, your Danish language is simply not loaded.

"Joomla article feature is enough without K2"

Pure nonsense I say...

In any case, don't be put off by a bad template provider. Joomla is a great *platform* and K2 is the best content management app for it.

Now, for some reason, when you add your YouTube URLs in the "Media" tab of the K2 item form, as you imply, these videos don't work in the frontend.

The solution is probably easier than you think: the relevant option in K2 that controls the display of media is simply disabled.

So edit your category settings in K2 and at the 3rd tab with all the display settings, scroll down to the "item view" options section. In there, look for the "Media" option and make sure it's set to "Show", as in this screenshot: jmp.sh/Ikf4Xmj

That's it.

Did you re-instate Protostar in Joomla as I mentioned previously?

Just load your site with the Protostar template (in which case, K2 loads its default template overrides): www.kolding-netavis.dk/dr-saetter-fokus-pa-genforeningens-100-arsdag?template=protostar

This will allow you to test if your template has overrides for K2 and if so, if they are missing content elements.