Well that is up to the client/user of PrintProvider. E.g. Heron's PrintPreview UX uses i18N:
https://code.google.com/p/geoext-viewer/source/browse/trunk/heron/ux/printpreview/lib/GeoExt.ux/PrintPreview.js (outputFormatText).
I was following the pattern of the other capabilities (dpi's etc).