language: fix bug where canonicalization is not idempotent

This currently only happens for
	Darkhat (drh) -deprecated-> Halh Mongolian (khk) -macro-> Mongolian (mn)
Added test to verify this property for all languages.

Change-Id: I4a2fb98d436fa88865872b9c62ed5f3114d15de5
Reviewed-on: https://go-review.googlesource.com/13072
Reviewed-by: Nigel Tao <nigeltao@golang.org>
2 files changed