In the U.S., even the most critically lauded subtitled films rarely make a dent at the box office. But this summer, the elegant French-language thriller “Tell No One” has been inexplicably marshalling audiences to art-house theaters and yielding higher per-screen average returns than Hollywood mega-musical “Mamma Mia!” The secret? Positive word-of-mouth and the fact that “ticket buyers seem to forget they are watching a foreign-language film.”