I’m not the Ghost team. 
(Sorry for replying anyway!)
I think items 1-3 should be covered by the translation efforts underway for Portal.
I’m guessing that what Ghost Pro support told you (should have told you?) was that those strings aren’t currently wrapped. I submitted a PR a year ago that fixed the /month problem and several other missing strings, but it didn’t get merged. (It’s possible that some other PR duplicated mine - I don’t know.) I’d assume that the goal is to get everything translatable in Portal.
4 - the search bar is a separate app (sodo-search). If you add a custom search integration, it can use whatever language you like. There used to be lots of themes that included their own search functionality, but with the addition of sodo-search, most people don’t use it any more. Here’s an example: GitHub - jamwise/ghostHunter: A Ghost blog search engine
5- if this is in your theme, you can change it. If this is in Portal, you can’t.