In Drupal 8.5.8, if you go to localize.drupal.org and download a translation or attempt to translate Drupal Core, you will find that the string "Basic page" is in the POT/PO database.
However, this string is missing from 8.6.2, and actually all 8.6.x releases on localize.drupal.org. It should be there -- it is still the displayed name of the Standard install profile's 'page' content type.
I will try to investigate why this is... one of my tests for the User Guide caught the problem.
When I export the Spanish translation from localize.drupal.org, this is what I get for 8.5.8 in the .po file:
#: core/profiles/demo_umami/config/install/node.type.page.yml:0; core/profiles/standard/config/install/node.type.page.yml:0
msgid "Basic page"
msgstr "Página básica"
This is missing from 8.6.x exports. In fact, the text "core/profiles/standard/config/install/node" doesn't appear in the .po export at all, although there are other .yml files listed from core/profiles/standard/config/install. Just not node types. And they definitely have 2 translatable strings in them: the content type name, and the description. Both are in the 8.5.8 file, and both are missing in the 8.6.2 and other 8.6.x files.