Filtering search results by language is broken

[adamwoodnz]

Steps to reproduce:

1. Navigate to an archive, eg. https://learn.wordpress.org/courses/?new-theme=1
2. Enter a search term that you can see in the title or excerpt of one of the cards, eg. 'friendly' exists in the excerpt of the Beginner WordPress User course
3. Submit the results and you should land at https://learn.wordpress.org/?s=friendly&post_type=course with results.
4. Filter the results by language. At present Italian is the only option.

Expected: No results as all the courses from the search are in english

Actual: The same result set as without the filter

With search, no language	With search, and language

Note that filtering all courses by language does work https://learn.wordpress.org/courses/?language%5B%5D=it_IT&post_type=course

[pkevan]

This is slightly complicated for a couple of reasons:

• the indexes for search are all classified as en language, I believe you can create a language specific index, but unsure how that is done
• language is stored in the site DB as a post_meta (language), but so you could potentially use this, but only if the meta is whitelisted in JP search so the request can be adjusted to include this