Enable localizations for credits and site navigation; add them for de, es, fr, it

Introduces the possibility to define localizations for credits and site navigation. Uses `_config.yml` to define localizations and falls back to `en` when no localizations have been defined for a specific language.

This also fixes a problem with navigation links to nonexistent subpages (`/de/security`, `/es/security`). These links can simply be omitted in the localized navigation bar.

Localizations for `de`, `es`, `fr`, and `it` are provided.