[improvement] Translation: automatic and manual language selection

Problem description:

https://github.com/swagger-api/swagger-ui/blob/1f2a5d7dd74131f0e9886b0f137ac20165aa90d2/dist/lang/translator.js#L6-L8

* To enable translation you should include one of language-files in your index.html
* after <script src='lang/translator.js' type='text/javascript'></script>.
* For example - <script src='lang/ru.js' type='text/javascript'></script>

This is unacceptable in today's internet. Every end user should see his language.
So:

1. swagger-ui should provide some query parameter (e.g. lang) to set set user's language and apply corresponding translation. (This is needed for integration purposes.)

2. If the parameter is not specified, swagger-ui should determine user's language automatically using the following variables (actually, all of them in the specified order) and apply corresponding translation:

   1. navigator.languages
   2. navigator.language
   3. navigator.userLanguage

3. End user should be able to change the language using swagger-ui.

Additional information:
swagger-ui version affected: commit 1f2a5d7
swagger file: not required

Start with dist/lang/translator.js, especially the documented language-file loading flow, and inspect the index.html integration described in the issue. Trace how translations are currently selected and loaded. Done means supporting an explicit lang query parameter, the listed browser-language fallbacks, and an end-user language change while applying the corresponding translation.