Ouch, this is a regression from c339bb6e109dc0ee493292c0b91841faff7c033c, really sorry about that.

As a workaround, it works when you provide a 'descriptionlang' parameter too: https://commons.wikimedia.org/w/index.php?title=Special:UploadWizard&campaign=wlm-ru&id=5910051000&id2=&descriptionlang=ru&description=Transfiguration_Monastery&categories=Transfiguration%20Monastery%2C%20Solikamsk&uselang=ru

I'll get this fixed tomorrow.

Change 258767 had a related patch set uploaded (by Bartosz Dziewoński):
mw.UploadWizardDetails: Don't fail when 'description' is given, but 'descriptionlang' isn't

https://gerrit.wikimedia.org/r/258767

@matmarex: Thanks! Does it mean that &descriptionlang= will automatically generate the relevant language template? Currently, we add this template using the code in Campaign:wlm-ru? Which way is better?

I'm not sure what you mean. Setting "&descriptionlang=ru&description=Transfiguration_Monastery" will prefill the description fields to generate "{{ru|Transfiguration_Monastery}}" (but allowing the user to change the description text and language). I don't see where in https://commons.wikimedia.org/w/index.php?title=Campaign:wlm-ru&action=edit this is configured.

@matmarex: Yes, you are right, it is not there. However, &uselang=ru set Russian as default language, so that every description was processed as {{ru|description}} unless users opted for a different language. I guess that &descriptionlang= is redundant in this case

Change 258767 merged by jenkins-bot:
mw.UploadWizardDetails: Don't fail when 'description' is given, but 'descriptionlang' isn't

https://gerrit.wikimedia.org/r/258767

Change 259006 had a related patch set uploaded (by Thcipriani):
mw.UploadWizardDetails: Don't fail when 'description' is given, but 'descriptionlang' isn't

https://gerrit.wikimedia.org/r/259006

Change 259006 merged by jenkins-bot:
mw.UploadWizardDetails: Don't fail when 'description' is given, but 'descriptionlang' isn't

https://gerrit.wikimedia.org/r/259006