Use the correct language for transliteration

In the transliteration processor, we currently just use the default language as the language of the transliterated text. This will of course be correct for unilingual sites, but is rather arbitrary for multilingual ones.
Instead we should:
- at index time, use the item's language;
- and at search time, maybe check if the query specifies a definite language, and only fall back to the site default as the last option.