Explicitly set locale fallbacks

What does this MR do?

With a recent change in i18n, default language is not included in fallbacks by default. This causes that MissingTranslationData exception is raised both in development and production mode.

This patch sets explicitly fallbacks language to english which assures that english is used for missing translations.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/54274

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.