Autocomplete: Multi-lang search (based on user's lang)

Transcription of https://github.com/pelias/api/issues/127#issuecomment-490002821

What is this for ?

We want Pelias to send responses to queries written in other languages than English.
For example, a Dutch looking for Parijs (Paris in Dutch) will get Parijs, Frankrijk.

What should we do ?

• Use multi_match in autocomplete queries (done in #1300)
• Should search in name.$LANG index with higher boost
• Should search in name.default index with standard boost
• Should search in name.en index as fallback with lower boost (when $LANG is not en)
• Should return name.$LANG when available, default otherwise (done in #1301)
• Found something for matching parent e.g Parijs, Frankrijk.

Some use cases

cc @mihneadb Have you started working on it ? I can take the task if you want 😄

Start from the autocomplete query and response handling in the Pelias API, then reproduce the listed Parijs and Londre cases using the language values shown. The completed checklist items are already covered by #1300 and #1301; done means resolving the remaining parent-matching case with the expected localized result.