I'm sure I read somewhere that there is an English version of CHDK available somewhere but I can't find it, my schoolboy German is not up to reading the menu items.
There is currently a problem with server that provides the English autobuild.
You should be able to reset the language to the current camera language in by entering alt mode (should be the play button on sx260, "Default script <alt>" will appear at the bottom of the screen), and run the default script by fully pressing the shutter button. You could also use the user manual https://chdk.fandom.com/wiki/CHDK_1.5_User_Manual
to help navigate the german menus to the language setting.
Alternatively, you can download an English build from https://build.chdk.photos/
and use stick to install it as a custom build.
When I tried reinstalling CHDK using Stick I noticed my antivirus software Bitdefender flash an alert on screen and on investigation I foud a message saying that a threat had been detected and quarantined. I didn't understand the information it was giving me but to rule out coincidence I ran Stick again and once more the alert popped up.
To cut a long story short, with Bitdefender Support help I removed all traces of their software and rebooted the PC. I deleted the old copy of Stick, downloaded a fresh copy and ran it againand itt completed with any problems. However, when I tried the camera with the newly prepared card, although the CHDK spash screen was displayed at startup the camera still will not save RAW/DNG files.
I don't know what else I can do so I am in desperate need of some expertise.
If the CHDK splash screen shows, then CHDK is running, and you should just need to enable raw and DNG in the CHDK raw settings.
I don't know what to say about the bitdefender issue, false positives aren't unheard of, but if the CHDK logo is showing, it probably isn't related to your CHDK problems.