Fix/translatable strings in theme json

[d-alleyne]

This is a backport of WordPress/gutenberg#66675

Trac ticket: https://core.trac.wordpress.org/ticket/62728

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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Props dalleyne.

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[github-actions]

Hi @d-alleyne! 👋

Thank you for your contribution to WordPress! 💖

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[github-actions]

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[audrasjb]

This looks good to go, thanks.