This was discussed here:
Some Plugin Failures (Text to Speech/Google Translate) - FireDragon
The problem setting in /usr/lib/firedragon/firedragon.cfg is:
defaultPref("services.settings.server", "https://%.invalid") // Set the remote settings URL (REMOTE_SETTINGS_SERVER_URL in the code)
If that is removed or commented out translation works.
Also once the required languages are downloaded the setting can be reapplied and translation still works for languages already downloaded.
You can download all languages at once in Settings.