The reason for this is that when you search for a term, the system doesn't just return exact matches, it also returns 'stemmed' matches: words that share the root of the search term: in English, a search on "depart" also returns results that contain "department", "departing", etc.
But this kind of stemming is language dependent: in different languages, the same root letters would be associated with a different set of stemmed words, or maybe none if the word is not part of that language. So the total search results are different depending which language you choose.
