Search localization

[jamesamcl]

For the homepage search we originally envisaged that it would search in all languages. However in practice this creates a lot of noise because the European languages are so similar. This is from the HP internationalised version from @matentzn :

(I already filtered the jump to to only show English strings, but the autocomplete list still has everything.)

We should probably default the home search box to English (or the language of the user's browser?) with options to switch to different languages. Also, the lang option in the API only filters the results, not the language you are searching in. We may need to have both search language and result language.