I would like to use two languages, and testing it works fine. But I can't add a translation for tags, is that right?

Yes, correct, tags are unique keywords different to titles and such as they are used in the url to search directly. If you neede two languages you need to use separate ones for each language. Or maybe combing them like "

Then the most simple solution is to just use both languages/tags, like cats/katzen and houses/häuser etc. It's not too annoying for visitors I think when not using too many tags for an image.

Good, then remember to set the search options for tags to partial as it otherwise might not work as expected if searching via the search form.

(German terms or is it in Dutch really the same?)

Ok, thanks for that tip!

Thanks for the tip acrylian, another mark ticked off.
I'll be using method #2.