Showing only (or prioritising) same-language results helps with relevancy. A German speaker who searches for “gift” is not looking for ideas for presents, and an Italian speaker who searches for “peperoni” is not looking for the American variant of salami.

A further problem, though, is that search engines try to understand the query and the content searched at a deeper level than mere keyword matches. For example, they may try to pick up synonyms (if I search for “movies in <local area>”, perhaps results for “films” might be helpful too) and associations.