Why does Magento 2 suspend inline translation while sending a transactional email?

Inline translation is disabled when sending an email because, when inline translation is active, any translatable printed text (__('Text here')) goes through Magento\Framework\Phrase\RendererMagento\Framework\Phrase\Renderer\Inline::render

This method adds some markup around the text {{{ and }}} that is used later by some javascript code that interprets them and shows the nice little buttons and popups for inline translation.... but....

This works only if you are on the website in question and that js is loaded.
Since phtml templates can be used in email templates, if inline translation is enabled, you would get the texts in the email like {{{ some text here }}{{ other text here }}} because the special markup will not be interpreted.
So this is a security measure so you don't screw up your email tempaltes

When in-line translation is allowed public function string will translated accordingly. So you need disable it and resume later.

This also prevent issues with improperly enabled inline translation.

Adding function for suspend inline translation was related with conflicts, please read :

https://github.com/magento/magento2/pull/11320

https://www.absolute-design.co.uk/services/magento-ecommerce/how-can-i-fix-magento-translation-conflicts/