language/display: add Formatter form

This should probably be the preferred form down
the road.

Use translation for “und” instead of “root” for
language.Und.

Not the most efficient implementation, but good
enough for now.

Change-Id: I2dfae579704b4afc883e26b38a05661477d81790
Reviewed-on: https://go-review.googlesource.com/61830
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>