The default language of votewiki in production is changed at least for a few weeks every year when votewiki is used to run the fawiki elections (in Persian). The root cause of that is a technical debt in that users are referred to votewiki from fawiki, yet MediaWiki-extensions-SecurePoll does not have a way to change the interface language for that user session to the language of the referring wiki. Since showing an English interface is not reasonable for a fawiki election (i.e., a community where not all users know English), a temporary change to the interface language is used instead.

Anyway, this makes sense for the production wiki, but not for the beta cluster wiki. That wiki's interface language setting should be independent, and possibly should always be English, so that those who need to test SecurePoll in a WMF-like environment can do it easily.