How can i add tags to multiple images at the same time in RawTherapee? If i mark some images in File Browser, there seems to be no way to edit IPTC Data for this photos together. I cant believe, that i have to open every photo in a separate view to edit metadata only for this single photo.
RawTherapee isn’t really designed for tagging images. There is a proposal for batch editing tags though: Batch edit of IPTC tags by Bezierr · Pull Request #6851 · Beep6581/RawTherapee · GitHub.
Have you tried using a profile? Open one photo, edit the IPTC data to your liking, then save that profile in RTs profile directory, named for example my_iptc.pp3.
Then select other photos in the file browser, right-click and say Profile operations, Apply profile, My profiles, my_iptc.
Is that what you’re looking for?
For quicker workflow just ctrl click the “copy current processing profile to clipboard” and check the metadata box.
Then use the paste processing profile button on a range of files selected in File browser view.
I may be missing something but doesn’t the .pp3 apply all of the image adjustments, not just the IPTC meta-data?
P.S. XnView MP can apply the same IPTC only to all selected files in it’s browse view - I just did it. 
The pp3 can contain a single setting. Thats critical to working with Rawtherapee imho. Ctrl click on the save profile icon and check the box you need.
OK, thanks!