On Book’ component all alerted lines are set into read-only mode. And read-only is expected, I remember we discussed it previous time why some lines are set for read-only, Empty translation values can't be completed in forum, plus checking code of platform the comment explains it. Unfortunately in Book’ code I’m not finding it book-versions/application-book-versions-common-ui/src/main/resources/BookVersions/Code/Translations.xml at b3ffa95233aad5899394285ab8700d10c8451ac1 · xwiki-contrib/book-versions · GitHub.
At least you mentioned them here 
with this post