The problem with Language Transfer is its very limited language selection and its format.
Duolingo allows reading, writing, listening and speech (last two can be disabled if unsuitable in your context), and it does not impose daily limits. I’ve yet to find an alternative app that does all 5 of those things.
With manifest v2, extensions could block the content however they wanted, reading and modifying DOM as they see fit.
Google claims that it is a security risk, so with manifest v3, extensions can only create and give the browser rules and the browser itself will block content based on them. The rules have a limit in size and capabilities.
If that was still not clear, try thinking of unrestricted SQL access vs a UI for modifying a database.