I think this is similar to an issue others are having with offline voice typing with GBoard. Obviously, since these apps are closed-source, we cannot know for sure what's going on.
However, my best guess is that the app checks system info like whether airplane mode is on, wifi is connected, etc. (some connectivity status info is accessible by apps). Since the system reports that it's connected to the internet, the app tries to do translation online. It'll only do offline translations if the system reports the system is offline.
Since network permission is an added GOS feature, apps can't cleanly detect whether they have access or not using a system API. GT and GBoard clearly don't fall back to offline data if there's an issue with the apps' connectivity.
I'm sure Google knows people block their apps' network access with VPNs or firewalls, but I'm also sure they don't care about our app experience because we're "DeGooglers," so they don't bother to make it work for us.