Improve accent position for Vietnamese.

See example 42 in the fontspec documentation.

Change-Id: I65d3cfeb14d083b2504a7fc844b6392e100750e4
Update multiwiki test case to include korean text.

The original pull didn't follow redirects, so the korean article was empty.

Change-Id: I35bf7a9f7b1b995694e4e603a7270e85c93be165