One of the most frequently received feedbacks about the mobile app is that it only searches in one language. To switch to a different language the user must leave go to the preferences. It happens to me all the time, because I use three languages; I know the app well and it annoys me quite a bit, and a lot of users don't know how to use the preferences.
A more cleverly designed search interface or language selector will be probably implemented in the future, but in the meantime the following simple solution can be implemented: a link saying "search in another language" can appear in the search results, somewhere near "show more results".
Version: 3.2 (iOS) / 1.2 (Android)
See also: T73136: Improve language selection