Multizim search requires that all selected books be in the same
language.
No new URL query parameter was introduced for specifying the intended
search language - `books.filter.lang` can be used for that purpose.
The server_search unit-test was updated to use a slightly cheating
library xml file where the language of example.zim was tweaked from "en"
to "eng" in order to match that of zimfile.zim. Note that this change
drops from the tested server two other goofy ZIM files corner_cases.zim
and poor.zim that have been/are included in ServerTest.
Veloman Yunkan