For example: there is a site in two languages, the user uses language #1 and publishes an ad in language #1 with the title "iphone". The second user likes language #2 and searches for "iphone" but the search doesn't find anything because the ad is published in language #1. Below I have attached two skins from your demo site where the search in English does not return results if the site is switched to another language.

Simply said if seller decides to put listing in 1 language, search will work just for users using this language.
Considering you have 10 languages on site, using search on all fields may bring serious performance issue.
Anyway we will try to inplement such switch/setting in future