language: implement paradigm locales

This allows deleting parent distance.

This also fully implements allowing the user to
order dialects non-contiguously.

Change-Id: I09a8d21a6c6e18edc24db136b280ba960eb72370
Reviewed-on: https://go-review.googlesource.com/55911
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>