We have "Refresh" button in "Configuration -> WebSite -> Regional" section. That button copies translation of missing phrases and emails from other languages. That could prove useful, when somebody have added new phrases to only one of used languages and you don't want to copy-paste their translations by hand.
And what if we actually have one language without phrases for testing purposes. For example we can use it to test if all text from initial HTML markup were converted to phrases. In this case that "Refresh" button creates each phrase in that empty language too.
I propose not to create missing phrases for languages, where there are 0 translated phrases. Same could be applied to email events.