Setting and using a "language" property of an audio file in iTunes?
The set of available columns appears to be fixed, and there is no language column.  But the way to change the set of displayed columns is by contextual menu (right-click) on the column headers.  (At least in 12.1—Apple seems to take pleasure in making sure we can't ever get accustomed to any particular UI for iTunes.)

There is a "category" column which I will use.  Not ideal, but better than usurping BPM or grouping.

Update: Changed my mind.  I can put lang: es; in Grouping and search for that in a "smart playlist."  That still allows me to put other things in Grouping.