Revert "Merge pull request #148 from dimagi/dont-delete-unknown-lang"

This reverts commit e4ae887823df5846744cb0c8430ad3b14d42f90e, reversing
changes made to ded13b64b2c2d3d0626ae18f59152e42c6150054.
improve warning for language that will be deleted
Merge pull request #165 from dimagi/vellum-languages-warning

Vellum languages warning