Bookmarks: Language option missing from search and filter interface

Example URLs
Bookmark Search page: https://test.archiveofourown.org/bookmarks/search
Bookmark tag page: https://test.archiveofourown.org/tags/Suits%20(TV)/bookmarks
User bookmark page: https://test.archiveofourown.org/users/testy/bookmarks
Collection bookmark page: https://test.archiveofourown.org/collections/yuletide/bookmarks

The problem

We index bookmarks based on the bookmarkable item's language, making it possible to search for bookmarks of works in another language, e.g. Russian:

• In Elasticsearch 0.90, you can do this by entering bookmarkable_language_id: 30 in "Any field" on the Bookmark Search page, or in "Search within results" on a tag, user, or collection bookmark page

• In Elasticsearch 6, you can do this by entering language_id: 30 in "Any field on work" on the Bookmark Search page, or in "Search within results" on a tag, user, or collection bookmark page

However, there's no way to select a language on either the filters or the search page.

The solution
On Elasticsearch 6 – no need to implement it on 0.90 – we should add a "Work language" select menu to both the filters and the search page.

• On the filters, it should be between "Search bookmarker's tags and notes" and "Bookmark types" (to mirror its position of after the search field on work filters)

• On the search page, it should be between "Bookmarker's tags" and the checkbox for "Rec"

  • If is implemented, it can be before or after the field for selecting the type

There should also be automated tests for these fields.

Notes
Only works – not series or external works – have languages set, so if you specify a language, the results will only be works.