How to change the language of strings like "read more" and "submitted by"? I've managed to create a website with Drupal 8.3.7.

How do I get Drupal to print things like "Read more" in views and "Submitted by", "Wednesday, August 23" in an article or t("something supposedly translated") in another language?

I'd like the administration interface to remain in English. I'd like to avoid a language prefix in URLs (yes: /mycurrenturlalias, no: /XX/mycurrenturlalias) but at this point I really don't even care anymore.

I've tried installing the Language module, adding a new language, setting it as the default and only language, installing some translation modules and fiddling with their settings. I've broken the site in some interesting ways but what I haven't gotten is a single translation of those strings anywhere. I must be doing something horribly wrong, but please ignore that and just tell me what should work.
Install the core modules Language and Interface Translation 
Add other language (admin/config/regional/language)
Configure the Selected language setting (admin/config/regional/language/detection)
To have a different language in admin enable the detection method Account administration pages
Go to user/1/edit and in Language settings section select your Administration pages language
Translate your strings (admin/config/regional/translate)