The MobileFrontend's search functionality displays the `mobile-frontend-search-content-no-results` message (currently set to //'''{{int:mobile-frontend-search-content}}''' to see if this phrase appears anywhere.//), using an embedded `{{int:mobile-frontend-search-content}}` to insert a bolded //Search within pages// phrase.
However, the construction (together with the message definition) causes the embedded `{{int:}}` to use the content language of the wiki instead of the user's UI language like the rest of the message. That means the users sees a bilingual mixed message whenever she uses a different UI language than the wiki content language.
For instance: go to https://cs.m.wikipedia.org/wiki/X?uselang=en and write some nonsense to the search box. You'll get "No page with this title. //Hledat v obsahu stránek// to see if this phrase appears anywhere." with a Czech translation of the subphrase within the English message. (Or vice versa: go to https://en.m.wikipedia.org/wiki/X?uselang=cs and write some nonsense there, and you'll get "Neexistuje žádná stránka s tímto názvem. //Search within pages//, zda se tento text někde neobjevuje.")
This bug have probably been reported already at T74985, which is now //Closed, Invalid//. Not sure if something changed since then or not, but this //is// definitely a bug, so the resolution does not make much sense to me.