Issue #12658 Add a deprecation note about Translator's setFallbackLocale...
minor #12671 Issue #12658 Add a deprecation note about Translator's setFallbackLocale... (javiernuber)

This PR was squashed before being merged into the 2.7 branch (closes #12671).

Discussion
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Issue #12658 Add a deprecation note about Translator's setFallbackLocale...

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #12658
| License       | MIT
| Doc PR        |

Commits
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cd05026 Issue #12658 Add a deprecation note about Translator's setFallbackLocale...