docs: Updates to N Preview multilingual support page

Continues work from changelist http://ag/874044
Misc. fixes to doc, including changing the release name (from "N
Developer Preview" to just "the Preview") after first use.

See first comment for doc stage location.

bug: 27048604
Change-Id: Ib00249e63d4649b834937f5dfe51e115c9472525