[i18n/l10n] Multi-lang sites: Translateable navigation labels & site data

[heyakyra]

Issue Summary

For proper internationalization, all site-wide data including navigation labels should be translatable and it should be possible to load that data based on either the route or the language of the post being viewed. Currently, there is a confusing misalignment of theme translation features and multi-language site features. See also: #8509

Data needing translations includes:

• Navigation labels
• Site title & description
• Site metadata

Data needing translations and/or a language property:

• Site pages
• Blog posts

The API Reference for Handlebars Themes' "translate" section says:

To ensure that your theme is fully translatable, two core templates must exist in your theme. Check the following templates exist:

pagination.hbs - exists in content/themes/mytheme/partials, copy the template

navigation.hbs - exists in content/themes/mytheme/partials, copy the template

While there are instructions for translating pagination at the end of the section, translations within locale files do not apply to navigation labels by default and there are no instructions for achieving this.

The section on subexpressions implies but doesn't clearly say whether (and if so, how) this could be used to translate arbitrary text returned by handlebar expressions.

[stale]

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