To not over-complicate things, we could alternatively think of a neutral wording that works for both Items and Properties (e.g. "Please provide a description in another language once this entity is created."). Just sharing my thoughts here, it's your decision, @Arian_Bozorg.

We will need to inject the entity type as a message parameter (or create separate messages per entity type).

IMHO splitting this into two separate messages (item/property) seems simplest, and more readable to users than using generic phrasing like “entity”.

I agree with Lucas here, I also feel that "entity" seems not specific enough in this instance

Would it be reasonable to change the message so that it doesn't mention the entity type at all? For example, something like this:
"Please provide a description in another language after creation"

I'm guessing there's a reason why it's not like this already. Maybe this would be harder to translate, or is less clear?

Although both can work, I think the specificity of having 'Item' or 'Property' removes any ambiguity of the message.

Change #1052317 had a related patch set uploaded (by Audrey Penven; author: Audrey Penven):

[mediawiki/extensions/Wikibase@master] [WIP] MUL: fix descriptions not supported for Special:NewProperty

https://gerrit.wikimedia.org/r/1052317

Change #1052317 merged by jenkins-bot:

[mediawiki/extensions/Wikibase@master] MUL: fix descriptions not supported for Special:NewProperty

https://gerrit.wikimedia.org/r/1052317

Can be tested here: https://test.wikidata.org/wiki/Special:NewProperty?lang=mul