In the QIIME 2 2018.2 release, Metadata now has explicit support for UTF-8 files and Unicode characters. If a non-UTF-8 metadata file is used, a more informative error message is displayed.
There are a number of other changes to QIIME 2 Metadata in the 2018.2 release. See this forum announcement for details on what changed, as well as the updated Metadata tutorial. 