Fix numeric translation keys being dissolved by array_merge

[tdgroot]

Description (*)

When creating translations like "100","Hundred", the key "100" will be dissolved by array_merge when combining all the dictionaries from all language packs. For example, the new key of that translation could be 1, 2, etc.

Manual testing scenarios (*)

1. Create a translation like "100","Hundred".
2. Check if if the key "100" translates to "Hundred".

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 @tdgroot. Thank you for your contribution
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[magento-engcom-team]

Hi @VladimirZaets, thank you for the review.
ENGCOM-3343 has been created to process this Pull Request

[magento-engcom-team]

Hi @tdgroot. 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.