CRM_Core_I18n - Provide a better label for new/unknown locales

CRM_Core_I18n - Provide a better label for new/unknown locales

Overview
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Suppose you add new/unrecognized data files in the `l10n` folder - and then
you navigate to the screen `civicrm/admin/setting/localization?reset=1`.

Before
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The new language appears in the admin UI as a blank item.

Every page in the app displays a warning about the unrecognized locale.

After
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The new language appears in the admin UI with a placeholder name (based on the code).

The warnings are not displayed.

Comment
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We've just had an issue where a new language was added to the `l10n`
data-set, and then all automated test-suites for all versions (incl `5.25`
RC and `5.24` stable) started to throw blocker errors on unrelated PRs.
This is because the `l10n` data is generally evergreen, but each branch/tag
of `civicrm-core` has the list hard-coded in multiple spots.

This patch tries to make the symptom less painful when mixing/matching
different revisions of `civicrm-core.git` and `l10n.git`.