Thanks for the feedback @Bennylin,
Encouraging to produce quality content is really important. This is one of the main goals for the next version of Content Translation, which is the main focus the team is working on now.
In the new version we plan to improve the way that the tool warns users when they they translation has too much unmodified text, allowing communities to define the threshold and making it easy to filter those articles that were created despite the warning. In addition, for cases where the level of unmodified automatic translation is extremely high we plan to block publishing those translations.
We think the proposed tools will help, and we'll need feedback from the communities to adjust the right thresholds. Changing the default translation service does not seem as effective since it will make things more complex to good-faith users, while vandals can still find their way.
While we provide support to the above mechanisms, some useful tools are the analytics page where you can identify peaks of deleted articles and the filters to target articles created with Content Translation.
I hope this is useful.