According to the page "Register a string with the Translate interface UI",, you have to visit a page containing an template-defined untranslated string using a foreign-language URL in order for that string to be findable on /admin/config/regional/translate/translate. There is no mention of this on /admin/config/regional/translate/translate or on /admin/config/regional/translate/i18n_string.

I suggest that on /admin/config/regional/translate/i18n_string, that information to this effect be added as a third bullet point.