[UX] Display translated system.core configuration values

[VasasA]

Description of the bug

The Locale module can translate the values of the system.core.json settings defined by "_config_translatables". These are only displayed if the getTranslated() method is used and not get().

"_config_translatables": [
        "anonymous",
        "site_name",
        "site_slogan",
        "maintenance_mode_message"
    ],

Steps To Reproduce

To reproduce the behavior:

1. Install Locale module.
2. Add a new language. Make it the default language. (admin/config/regional/language)
3. Translate (admin/config/regional/translate/translate) this string: [site:name] is currently under maintenance. We should be back shortly. Thank you for your patience.
4. Put site into maintenance mode. (admin/config/development/maintenance)
5. Check the maintenance message with the anonymous user.

Actual behavior

The maintenance message is not translated.

Expected behavior

The maintenance message is translated.

Additional information

• Backdrop CMS version: 1.32.x-dev
• [Related discussion](#Backdrop > Config and translation @ 💬)

[indigoxela]

Nice catch @VasasA, many thanks for providing a PR.

One nit pick: the magic Fixes in front of the issue link in your PR description's missing, so the PR doesn't show up in the issue header here.