Fix translations of category design theme not being applied

[cezary-zeglen]

Up-port of #17854

Description

Currently if you enable custom design theme for category the translations of selected theme are not used. This commit fixes the problem.

Fixed Issues (if relevant)

#17625

Manual testing scenarios

(Copied from #17625 )

1. Enable Magento_Blank for the website
2. Create a category and set Magento_Luma as it's theme
3. Add a translation to the /i18n/ directory of Magento_Luma
4. Flush caches
5. The translation is now not applied.
   Commit changes point 5. so the translation is being applied

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @cezary-zeglen. Thank you for your contribution
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[orlangur]

Cc: @slavvka

[magento-engcom-team]

Hi @orlangur, thank you for the review.
ENGCOM-3365 has been created to process this Pull Request

[magento-engcom-team]

@cezary-zeglen thank you for contributing. Please accept Community Contributors team invitation here to gain extended permissions for this repository.

[magento-engcom-team]

Hi @cezary-zeglen. Thank you for your contribution.
We will aim to release these changes as part of 2.3.1.
Please check the release notes for final confirmation.