No search results returned for valid ISO 639-1 codes

[logological]

According to the search guide, the language field can be used to search for records according to an "ISO639 two or three letter language code". In my experience, searching for a two-letter (ISO 639-1) code never returns any results, even if there are records in that language, but searching for the corresponding three-letter (ISO 639-2 or 639-3) code does return the records in that language.

For example, a search for language:de returns the message "We couldn't find any matches for 'language:de'", but a search for language:deu currently returns 16,693 records.

I suspect that either the language search is broken because it doesn't handle two-letter language codes, or the documentation is wrong because two-letter language codes are not supposed to work in the first place.

Possibly related to #802.