language: improve compliance of matcher

- fix und-TW bug
- remove region distance
- deemphasizes exact matches:
Most people are not fully bilingual so a language
in first position should be a strong preference.
User may still define bilingual support by explicitly
ordering second language among dialects

Change-Id: I2ee8721e11764370e4f937fd2ee979586b1aa1a5
Reviewed-on: https://go-review.googlesource.com/55750
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
4 files changed