If a translation is completely wrong, the Special:Translate interface provides a drop-down to the history to let a user revert it to the last-known good version (but it's a bit clunky compared to the whizzy Translate interface).
However, if there isn't such a version (first translation is the wrong one), deleting the translation is the right way to go - the drop-down should include a "Delete" entry. Note that bug 39415 blocks this.
Apparently it's also "known" that you can replace the contents with "FUZZY" which might be an alternative replacement for this enhancement request, as only a few power users would know this (and it's not clear how users would learn it).
Version: master
Severity: enhancement
See also: