Change name for Romansh locale

[swissspidy]

Original request: https://make.wordpress.org/polyglots/2016/10/20/name-change-request-for-roh-locale/

The name should be changed to "Rumantsch" as "Vallader" is only one of many regional idioms. However, roh stands for the overall Rumantsch language.

Note: Rumantsch Grischun is the government's attempt at a written language that unifies all these idioms. So WordPress translations would be done in this variant for example.