Customization: Translation slugs in JSON

[rvveber]

For the json to customize the theme, we could use
translation slugs instead of plain text for values.

Some reasons are:

1. It could simplify things by removing the need for default, fr, en and similar sub-keys.

2. It would also alleviate the burden of updating the full theme configuration when a new language is added, a process currently more involved than maintaining a flat JSON for translations.

3. This shift would also enhance modularity and better separate responsibilities.

4. It would allow translators of crowdin to properly translate the default values.

5. It would reduce the size of the theming file significantly - potentially improving payload transfer speed and overall reactivity of the app

6. It would allow nested translations to be integrated in the future, further reducing complexity and improving flexibility

Originally posted by @rvveber in #959 (comment)