Migrate spec/javascripts/locale/ to Jest

What does this MR do?

This MR migrates all spec/javascripts/locale/ tests to Jest.

unit testing frontend ~backstage

Closes #194243 (closed)

/cc @pslaughter

Open Topic

• I've switched from de to en language for the createDateTimeFormat test, since Node and hence also Jest doesn't have the full ICU set (see https://nodejs.org/api/intl.html#intl_internationalization_support). Since we don't need to test the Int object itself, this change should be fine. Alternatively, we could integrate the full ICU set for Node, see https://www.basefactor.com/javascript-es6-intl-not-working-properly-when-running-jest-tests

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team