Narrowing this to whether the right page still lands in the top 5 is the right scope, and it is the measurement most cross-lingual discussions skip past on the way to answer quality.
The thing I would watch in the numbers is that degradation tends to be uneven rather than uniform. Domain terms that are loanwords survive the language switch nearly intact, while everyday phrasing collapses, so a golden set weighted toward jargon looks far healthier than real user questions do.
If you extend the series, translating the query into the corpus language before retrieval is usually cheaper than a multilingual embedder, and it has the advantage of being inspectable when it goes wrong.