Browser language lookup is incorrectly using ISO 639-1 instead of BCP 47

### Problem

As I mentioned in https://github.com/internetarchive/openlibrary/issues/5997#issuecomment-3993278982, the language tags sent by browsers are based on BCP 47, not ISO 639-1, so doing ISO 639-1 based lookups is incorrect. This will not work for the over 300 MARC languages which don't have ISO 639-1 codes, disenfranchising those users.

In Wikidata, these can be found using the property [P305](https://www.wikidata.org/wiki/Property:P305) "IETF Language Tag". For languages which have ISO 639-1 codes, these will overlap, but most languages do not have an ISO 639-1 code.

### Reproducing the bug

* Expected behavior: Language specific searches like Random Book work for all MARC languages
* Actual behavior: Only ISO 639-1 languages work

### Context

- Browser (Chrome, Safari, Firefox, etc):
- OS (Windows, Mac, etc):
- Logged in (Y/N):
- Environment (prod, dev, local): prod

### Breakdown

Implementation Details (for maintainers)

#### Requirements Checklist
* [ ]

#### Related files

*

#### Stakeholders

* @cdrini
* @mekarpeles

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