Canvas language selector marks the default language with non-standard "(Default)" label and shows a stray separator without permission

Split from [#3591793 (closed)], given debates on whether to show the language selector for previewing other things that are not Canvas pages. Even thought that there are 2 problems with the Canvas language selector labeling and visuals:

1. A "Default" marker is appended which is different terminology from both core translation modules and Canvas Translate

The dropdown shows the site default language version as ("Default"). For all other translation UIs in Drupal, the original version of the entity itself is marked as "original" or "Original language". To be consistent with that, and given that recent Canvas releases allow creating pages in other languages, we should use "(Original)" instead.

Note the label needs to be keyed off the entity's original language (the first available translation), not the site default language. The site default language is not necessarily the language the entity was created in in Canvas at this point.

2. The horizontal line is shown even if the user does not have permission to edit the languages

The configure languages link is correctly missing but the horizontal line remains in this case, which is quite odd visually.