I've might have found a sollution for you aswell.
https://wordpress.org/support/topic/domain-mapping-with-translation-plugins
Workflow:
In "Domain Mapping" on your network admin select "Disable primary domain check".
Now all domains linked to a blog will be working. Keep in mind that this can result in duplicate content if you don't use multiple languages!
So if you have more client-sites running in this network you can't use the primary-domain function of MU Domain Mapping. Instead you'll need to redirect the non-primary domains in your server (add sites as an alias). With this workflow you won't need the add the non-primary domains in your wordpress network since the server is handling these redirections.
IMPORTANT: Make sure that the siteurl and the homurl is the same as your primary language domain!
Maybe not the best sollution since you can only do this if you need an Plesk of Cpanel in order to do this, but it works :).
What would be awesome is that MU Domain Mapping could create an update where you can select de "Disable primary domain check" for each blog...