Draft: Add pseudo English locale in test and development

⚠ WIP ⚠

What does this MR do and why?

Add a new locale "Pseudo English".

Generate this locale on-the-fly (during boot for both Gettext (locale/) and I18n (config/locale)) and use it as default in test to spot all ill-placed or missing translations. Mark all failing spec examples/contextes as TODO in spec/support/locale_todo.yml`` to fallback to default locale :en`.

This pseudo locale can be selected in development mode.

Take on #19484 (comment 220805837).

Inspired by !17757 (closed) and !17520 (closed).

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