[kmontrose]

Localization probably kills that, since this is displayed to users a not-insignificant amount of the time.

"9" is pretty well universal (I know, not strictly, but for software Arabic numerals are kind of assumed knowledge); "Nine" on the other hand isn't something you know without being able to read English.

(Having been through a pretty big localization project recently... this stuff sucks. So much. All your assumptions start breaking.)