🌐 feat(i18n): overhaul translation system & add languages (#145)

Revamp the existing translation system, simplifying
management and adding several new languages. The new system reads from
TOML files in the `/i18n` directory and improves template structures.
It also enhances customisation options and robustness by providing
fallbacks and modularity.

- Implement a new, streamlined translation macro.
- Load translations from `/i18n` TOML files.
- Remove redundant configuration requirements.
- Refactor templates to align with new i18n system.
- Add support for Hindi, Japanese, Russian, Portuguese, Chinese,
  Italian, German, Ukranian, Korean, and French languages.
- Credit Thomas Weitzel (@thomasweitzel) for inspiration.

theme.toml

@@ -19,14 +19,6 @@ homepage = "https://osc.garden"
 # be merged with user data, some kind of prefix or nesting is preferable
 # Use snake_casing to be consistent with the rest of Zola
 [extra]
-
-# Languages for your site.
-# Each should have a translation section above.
-# e.g. [languages.ca.translations]
-# language_name.ca = "Català"
-# language_name.en = "English"
-# language_name.es = "Español"
-
 # Enable JavaScript theme toggler to allow users to switch between dark/light mode.
 # Also enables automatic switching based on user's OS-level theme settings.
 # If disabled, your site will only use the theme specified in the `default_theme` variable.