Language::truncate(): don't chop up multibyte characters when input contains newlines

Language::truncate(): don't chop up multibyte characters when input contains newlines

To detect whether the truncation had chopped up a multibyte
character after the first byte, a regex was used. But in this
regex, the dot (.) didn't match newlines, so it failed to
detect chopped multibyte characters (after the first byte)
if there was a newline preceding the chopped character.

Bug: T116693
Change-Id: I66e4fd451acac0a1019da7060d5a37d70963a15a