[FrameworkBundle][Translator] Validate locales.
Add tests and more assertions
security #11830 n/a (aitboudad, Jérémy Derussé)

This PR was merged into the 2.3 branch.

Discussion
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Commits
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9e1bc22 Add tests and more assertions
101a3b7 [FrameworkBundle][Translator] Validate locales.