navbar: add a language chooser

[ronnychevalier]

Add a dropdown menu in the navbar when there is translations available
for a page.

It only supports currently supported languages from the i8n/ folder.
For each new translation in the i8n/ folder, a new mapping must be
added in native_names.yaml.

See #67

[gcushen]

Thank you very much for implementing all the remaining multi-lingual features from issue #67 😃

I agree that using Font Awesome's language icon makes a lot of sense. However, many of the most popular sites like Facebook and Microsoft use a globe icon to represent their language choosers, so I have updated the PR to be consistent with their approach.

Also, I have updated the native zh value to be consistent with how Google and Facebook refer to Simplified Chinese.

Perhaps it might be worth considering renaming lang in the path to i18n since this is all related to i18n?

[ronnychevalier]

Sure, all is fine for me :)

Thanks

[gcushen]

OK, do you want to rename data/lang/native_names.yaml to data/i18n/native_names.yaml or maybe even data/i18n/languages.yaml? Unless you think the original path is better?

[gcushen]

Thank you very much @ronnychevalier :)