Plain-text insource search should match interwiki links, but it doesn't when the prefix is omitted in the query.
Compare:
https://en.wikipedia.org/w/index.php?title=Special%3ASearch&profile=default& search=insource%3A%22A%2C+B+et+C+%28Le+Prisonnier%29%22&fulltext=Searchs&rbackend=CirrusSearch
https://en.wikipedia.org/w/index.php?title=Special%3ASearch&profile=default&search=insource%3A%22fr%3AA%2C+B+et+C+%28Le+Prisonnier%29%22&fulltext=Search&srbackend=CirrusSearch
Version: unspecified
Severity: normal