Not strictly true. Most translation departments have generic mailboxes for customers and people outside the department so that other people can pick up work when others are away. Their own email accounts would normally be used for internal email only.
Also, even a Welsh speaker may be unwilling to translate their own work - it isn't a good use of time, especially a large document with contributions from many sources. It goes to a dedicated department where it can be translated and proof-read.
It doesn't change the fact that the autosig should have been bilingual.