Add /language/ API endpoint

The new endpoint returns a list with language meta data for all ISO-639-1 codes. For each language the code, native name and localized name are returned. The language for the localized name is determined by the Accept-Language header.

Example

Request:

GET /language/
Accept-Language: de

Response (partial):

[
  {
    code: "ko",
    nativeName: "한국어",
    localizedName: "Koreanisch"
  }
]

Closes: #217 (closed)