Make {{#language}} consistent with {{#dir}} and {{#bcp47}}

Make {{#language}} consistent with {{#dir}} and {{#bcp47}}

Add the same no-arg options for language code that
{{#dir}} and {{#bcp47}} have, for consistency:
* `{{#language}}` will return the name of the *target language*
  (for articles, the content language; for messages, the user language)

The default value for the "in language" argument should be the autonym.
This was working previously but only via a baroque code flow path for
invalid language codes.  Make this a bit clearer and add tests.

Since non-autonym language code translations are added via the
[[Extension:CLDR]] in production, hook LanguageGetTranslatedLanguageNames
in the ParserTestRunner to ensure that we can test this.

Followup-To: Ice1c671c5b3cc077d2bb80ea5dc25c5eabbfeb36
Followup-To: I19c3e91a924e080f37dc95a0d4e61493583b533e
Change-Id: Ibf6e7f194cc056eadb48a5ad8e6d01a761d9351c