See https://www.mediawiki.org/wiki/Topic:S8k3rq6bbvw5x62z
The current source selector prevents users from selecting a language that is not enabled. This is by design, but it has the side effect that people cannot experiment using their language by directly changing the 'to'/'from' parameters in the URL.
Perhaps we need to pre-fill those manually entered languages in the source language selector rather than falling back to enabled languages?