Cannot split topic - see error logs in the topic body

Something strange is happening with error #2. The pluralization fails with English strings and on first glance they look perfectly fine:

https://github.com/discourse/discourse/blob/master/config/locales/server.en.yml#L1286-L1288

I think the following error is related to this:
https://meta.discourse.org/t/500-internal-server-error-when-trying-to-write-a-new-topic/37026