Page namespace was recently redefined on pawikisource (from English to local name) in T226959. However, it looked like this:

Note that in the URL the namespace is localized, but in the display title area, the namespace is still in English. After purging (action=purge was enough, null edit wasn't needed), this got fixed automatically.

Having to use a purge is always a sign that something is not working as it should/could. Filling this bug to inform ProofreadPage's maintainers.