Bug in [[Special:LanguageStats]]
Calling the page is tricky because the loading takes mostly a long time and a blank page is shown.
There was another thread and a ResourceLoader bug allegedly responsible of this, is it still happening?
Yes, it was still happening, as reported by malafaya, for the same problem as in the other thread (document.write or something, I believe) but in different circumstances. Purging always reproduced it.[1] The solution was "$wgResourceLoaderExperimentalAsyncLoading to false" (gerrit:62656).